The fullwidth environment starts a new page. It's fine for parts or
chapters, but not for a short table.
tufte-book puts the captions on the outer margin.
Here are two additions to your code: a new largetable environment for tables larger than the width of the text, and a new command \captionatbottom to work with largetable for the typesetting of the caption below the table.
It was tested on odd and even pages.
UPDATE (long captions)
Using largetable with normal width.

% !TeX TS-program = xelatex
\documentclass[a4paper,nobib,twoside,symmetric,justified,marginals=raggedouter, notoc]{tufte-book}
\makeatletter
\newlength{\oddshift}
\newcommand{\captionatbottom}{% from https://tex.stackexchange.com/a/229419/161015
\long\def@caption##1[##2]##3{%
\par
\addcontentsline{\csname ext@##1\endcsname}{##1}%
{\protect\numberline{\csname the##1\endcsname}{\ignorespaces ##2}}%
\begingroup
@parboxrestore%
\if@minipage@setminipage\fi%
\normalsize
@makecaption{\csname fnum@##1\endcsname}{\ignorespaces ##3}\par
\endgroup}
\long\def@makecaption##1##2{%
\vskip\abovecaptionskip%
@tufte@checkoddpage%
\ifthenelse{\boolean{@tufte@odd@page}}%
{\rlap{\parbox{\textwidth+\marginparwidth+\marginparsep}{\hskip0pt@tufte@caption@font##1: ##2}}}%
{\setlength{\oddshift}{-\marginparwidth-\marginparsep}\rlap{\hskip\oddshift\parbox{\textwidth+\marginparwidth+\marginparsep}{@tufte@caption@font##1: ##2}}}%
\vskip\belowcaptionskip%
}
\let\caption@tufte@orig@caption%
\let\label@tufte@orig@label}
\newenvironment{largetable}{%
\@tufte@checkoddpage%
\ifthenelse{\boolean{@tufte@odd@page}}%
{\begin{table}[!htbp]\begin{adjustwidth}{}{}\captionatbottom}%
{\begin{table}[!htbp]\begin{adjustwidth}{-\marginparwidth-\marginparsep}{}\captionatbottom}}
{\end{adjustwidth}\end{table}}
\makeatother
\usepackage{booktabs}
\geometry{% normal pages
paperheight =297mm, paperwidth = 210mm, %layout=a4paper
layoutheight =240mm,layoutwidth =170mm,
textheight=38\baselineskip,
headheight=\baselineskip,
left=15mm, % right will be the same because of symmetric
bindingoffset= 5mm, % add space for binding
textwidth=90mm,
top=14mm,
marginparsep=6mm, marginparwidth=40mm,
includehead,
layoutvoffset= 28.5mm,layouthoffset= 20mm, % 297-240 = 57mm 210-170 = 40mm ->> to center smaller layout in a4
showframe=true,
showcrop=true }
\renewenvironment{fullwidth}
{\newgeometry{% full width pages
layoutheight =240mm,layoutwidth =170mm,
textheight=38\baselineskip,
headheight=\baselineskip,
left=15mm, % right will be the same because of symmetric
bindingoffset= 5mm, % add space for binding
textwidth=136mm, % 90+ 6+ 40
top=14mm,
marginparsep=0mm, marginparwidth=0mm, %changed to full width <<<<<<<<<<
includehead,
layoutvoffset= 28.5mm,layouthoffset= 20mm, % 297-240 = 57mm 210-170 = 40mm ->> to center smaller layout in a4
showframe=true,
showcrop=true }
\fancyhfoffset[LE]{0pt}
\fancyhfoffset[RO]{0pt}
}
{\restoregeometry}%
\usepackage{kantlipsum} % dummy text added
\RequirePackage{showframe} % margin lines <<<<<<<<<<<<<<<<<<<<<<<<<
\renewcommand\ShowFrameLinethickness{0.1pt}
\renewcommand\ShowFrameColor{\color{red}}
\newcommand{\textls}[2][5]{%
\begingroup\addfontfeatures{LetterSpace=#1}#2\endgroup
}
\renewcommand{\allcapsspacing}[1]{\textls[15]{#1}}
\renewcommand{\smallcapsspacing}[1]{\textls[10]{#1}}
\renewcommand{\allcaps}[1]{\textls[15]{\MakeTextUppercase{#1}}}
\renewcommand{\smallcaps}[1] {\smallcapsspacing{\scshape\MakeTextLowercase{#1}}}
\renewcommand{\textsc}[1]{\smallcapsspacing{\textsmallcaps{#1}}}
\usepackage{fontspec}
\usepackage{libertine} %<<<<<<<<<<<<<<<<
\setsansfont{TeX Gyre Heros}[Scale=MatchUppercase]
\usepackage{xspace}
\newcommand{\monthyear}{%
\ifcase\month\or janvier\or f'{e}vrier\or mars\or avril\or mai\or juin\or
juillet\or ao^{u}t\or septembre\or octobre\or novembre\or
d'{e}cembre\fi\space\number\year
}
\newcommand{\blankpage}{\newpage\hbox{}\thispagestyle{empty}\newpage}
\usepackage[english,french]{babel}
\frenchbsetup{StandardLayout=true}
\addto\captionsfrench{\def\tablename{Tableau}}
\addto\captionsfrench{\def\figurename{Figure}}
\frenchbsetup{IndentFirst=false}
\usepackage{csquotes}
\begin{document}
\frenchspacing
\mainmatter
\part{First Part of the book}
\chapter{Introduction}
\begin{largetable}
\centering
\selectlanguage{french}
\footnotesize%
\begin{tabular}{llll}
\toprule
Fugue BWV 532a & & Fugue BWV 532 & \\
\midrule
mes. 1-27 & A & mes. 1-27 & Identiques \\
mes. 28-29 & & mes. 28-29 & Contenus différents\\
mes. 30-52 & & mes. 30-52 & Identiques \\
mes. 52-58 & B & mes. 52-58 & Entrée de pédale répartie entre les pieds et les mains\\
& & & dans la version BWV 532a \\
mes. 59-61 & & mes. 59-64 & \'{E}pisode différent\\
mes. 62-71 & & mes. 65-76 & Identiques, mais entrée plus courte dans la version BWV 532a\\
mes. 71-73 & & - & \'{E}pisode dans la version BWV 532a\\
- & & mes. 76-96 & Dans la version BWV 532, plusieurs épisodes et (courtes) entrées\\
& & & en \textit{do} $\sharp$ mineur/\textit{mi} majeur\\
mes. 74-98 & C & mes. 96-137 & Mêmes entrées mais épisodes plus longs dans la version BWV 532\\
\bottomrule
\end{tabular}
\caption{Différences entre les fugues BWV 532 et BWV 532a relevées par Peter Williams (1980). Différences entre les fugues BWV 532 et BWV 532a relevées par Peter Williams (1980).}
\label{tab:fugue532am}
\end{largetable}
\kant[11-12]
\begin{largetable}
\centering
\selectlanguage{french}
\footnotesize%
\begin{tabular}{llll}
\toprule
Fugue BWV 532a & & Fugue BWV 532 & \
\midrule
mes. 1-27 & A & mes. 1-27 & Identiques \
mes. 28-29 & & mes. 28-29 & Contenus différents\
mes. 30-52 & & mes. 30-52 & Identiques \
mes. 52-58 & B & mes. 52-58 & Entrée de pédale répartie entre les pieds et les mains\
& & & dans la version BWV 532a \
mes. 59-61 & & mes. 59-64 & '{E}pisode différent\
mes. 62-71 & & mes. 65-76 & Identiques, mais entrée plus courte dans la version BWV 532a\
mes. 71-73 & & - & '{E}pisode dans la version BWV 532a\
- & & mes. 76-96 & Dans la version BWV 532, plusieurs épisodes et (courtes) entrées\
& & & en \textit{do} $\sharp$ mineur/\textit{mi} majeur\
mes. 74-98 & C & mes. 96-137 & Mêmes entrées mais épisodes plus longs dans la version BWV 532\
\bottomrule
\end{tabular}
\caption{Différences entre les fugues BWV 532 et BWV 532a relevées par Peter Williams (1980). Différences entre les fugues BWV 532 et BWV 532a relevées par Peter Williams (1980).}
\label{tab:fugue532an}
\end{largetable}
\end{document}
Test of largetable inside fullwidth

% !TeX TS-program = xelatex
\documentclass[a4paper,nobib,twoside,symmetric,justified,marginals=raggedouter, notoc]{tufte-book}
\makeatletter
\newlength{\oddshift}
\newcommand{\captionatbottom}{% from https://tex.stackexchange.com/a/229419/161015
\long\def@caption##1[##2]##3{%
\par
\addcontentsline{\csname ext@##1\endcsname}{##1}%
{\protect\numberline{\csname the##1\endcsname}{\ignorespaces ##2}}%
\begingroup
@parboxrestore%
\if@minipage@setminipage\fi%
\normalsize
@makecaption{\csname fnum@##1\endcsname}{\ignorespaces ##3}\par
\endgroup}
\long\def@makecaption##1##2{%
\vskip\abovecaptionskip%
@tufte@checkoddpage%
\ifthenelse{\boolean{@tufte@odd@page}}%
{\rlap{\parbox{\textwidth+\marginparwidth+\marginparsep}{\hskip0pt@tufte@caption@font##1: ##2}}}%
{\setlength{\oddshift}{-\marginparwidth-\marginparsep}\rlap{\hskip\oddshift\parbox{\textwidth+\marginparwidth+\marginparsep}{@tufte@caption@font##1: ##2}}}%
\vskip\belowcaptionskip%
}
\let\caption@tufte@orig@caption%
\let\label@tufte@orig@label}
\newenvironment{largetable}{%
\@tufte@checkoddpage%
\ifthenelse{\boolean{@tufte@odd@page}}%
{\begin{table}[!htbp]\begin{adjustwidth}{}{}\captionatbottom}%
{\begin{table}[!htbp]\begin{adjustwidth}{-\marginparwidth-\marginparsep}{}\captionatbottom}}
{\end{adjustwidth}\end{table}}
\makeatother
\usepackage{booktabs}
\geometry{% normal pages
paperheight =297mm, paperwidth = 210mm, %layout=a4paper
layoutheight =240mm,layoutwidth =170mm,
textheight=38\baselineskip,
headheight=\baselineskip,
left=15mm, % right will be the same because of symmetric
bindingoffset= 5mm, % add space for binding
textwidth=90mm,
top=14mm,
marginparsep=6mm, marginparwidth=40mm,
includehead,
layoutvoffset= 28.5mm,layouthoffset= 20mm, % 297-240 = 57mm 210-170 = 40mm ->> to center smaller layout in a4
showframe=true,
showcrop=true }
\renewenvironment{fullwidth}
{\newgeometry{% full width pages
layoutheight =240mm,layoutwidth =170mm,
textheight=38\baselineskip,
headheight=\baselineskip,
left=15mm, % right will be the same because of symmetric
bindingoffset= 5mm, % add space for binding
textwidth=136mm, % 90+ 6+ 40
top=14mm,
marginparsep=0mm, marginparwidth=0mm, %changed to full width <<<<<<<<<<
includehead,
layoutvoffset= 28.5mm,layouthoffset= 20mm, % 297-240 = 57mm 210-170 = 40mm ->> to center smaller layout in a4
showframe=true,
showcrop=true }
\fancyhfoffset[LE]{0pt}
\fancyhfoffset[RO]{0pt}
}
{\restoregeometry}%
\usepackage{kantlipsum} % dummy text added
\RequirePackage{showframe} % margin lines <<<<<<<<<<<<<<<<<<<<<<<<<
\renewcommand\ShowFrameLinethickness{0.1pt}
\renewcommand\ShowFrameColor{\color{red}}
\newcommand{\textls}[2][5]{%
\begingroup\addfontfeatures{LetterSpace=#1}#2\endgroup
}
\renewcommand{\allcapsspacing}[1]{\textls[15]{#1}}
\renewcommand{\smallcapsspacing}[1]{\textls[10]{#1}}
\renewcommand{\allcaps}[1]{\textls[15]{\MakeTextUppercase{#1}}}
\renewcommand{\smallcaps}[1] {\smallcapsspacing{\scshape\MakeTextLowercase{#1}}}
\renewcommand{\textsc}[1]{\smallcapsspacing{\textsmallcaps{#1}}}
\usepackage{fontspec}
\usepackage{libertine} %<<<<<<<<<<<<<<<<
\setsansfont{TeX Gyre Heros}[Scale=MatchUppercase]
\usepackage{xspace}
\newcommand{\monthyear}{%
\ifcase\month\or janvier\or f'{e}vrier\or mars\or avril\or mai\or juin\or
juillet\or ao^{u}t\or septembre\or octobre\or novembre\or
d'{e}cembre\fi\space\number\year
}
\newcommand{\blankpage}{\newpage\hbox{}\thispagestyle{empty}\newpage}
\usepackage[english,french]{babel}
\frenchbsetup{StandardLayout=true}
\addto\captionsfrench{\def\tablename{Tableau}}
\addto\captionsfrench{\def\figurename{Figure}}
\frenchbsetup{IndentFirst=false}
\usepackage{csquotes}
\begin{document}
\frenchspacing
\mainmatter
\part{First Part of the book}
\begin{fullwidth}
\chapter{Introduction}
\kant[1]
\begin{largetable}
\centering
\selectlanguage{french}
\footnotesize%
\begin{tabular}{llll}
\toprule
Fugue BWV 532a & & Fugue BWV 532 & \
\midrule
mes. 1-27 & A & mes. 1-27 & Identiques \
mes. 28-29 & & mes. 28-29 & Contenus différents\
mes. 30-52 & & mes. 30-52 & Identiques \
mes. 52-58 & B & mes. 52-58 & Entrée de pédale répartie entre les pieds et les mains\
& & & dans la version BWV 532a \
mes. 59-61 & & mes. 59-64 & '{E}pisode différent\
mes. 62-71 & & mes. 65-76 & Identiques, mais entrée plus courte dans la version BWV 532a\
mes. 71-73 & & - & '{E}pisode dans la version BWV 532a\
- & & mes. 76-96 & Dans la version BWV 532, plusieurs épisodes et (courtes) entrées\
& & & en \textit{do} $\sharp$ mineur/\textit{mi} majeur\
mes. 74-98 & C & mes. 96-137 & Mêmes entrées mais épisodes plus longs dans la version BWV 532\
\bottomrule
\end{tabular}
\caption{Différences entre les fugues BWV 532 et BWV 532a relevées par Peter Williams (1980). Différences entre les fugues BWV 532 et BWV 532a relevées par Peter Williams (1980).}
\label{tab:fugue532am}
\end{largetable}
\kant[11-12]
\begin{largetable}
\centering
\selectlanguage{french}
\footnotesize%
\begin{tabular}{llll}
\toprule
Fugue BWV 532a & & Fugue BWV 532 & \
\midrule
mes. 1-27 & A & mes. 1-27 & Identiques \
mes. 28-29 & & mes. 28-29 & Contenus différents\
mes. 30-52 & & mes. 30-52 & Identiques \
mes. 52-58 & B & mes. 52-58 & Entrée de pédale répartie entre les pieds et les mains\
& & & dans la version BWV 532a \
mes. 59-61 & & mes. 59-64 & '{E}pisode différent\
mes. 62-71 & & mes. 65-76 & Identiques, mais entrée plus courte dans la version BWV 532a\
mes. 71-73 & & - & '{E}pisode dans la version BWV 532a\
- & & mes. 76-96 & Dans la version BWV 532, plusieurs épisodes et (courtes) entrées\
& & & en \textit{do} $\sharp$ mineur/\textit{mi} majeur\
mes. 74-98 & C & mes. 96-137 & Mêmes entrées mais épisodes plus longs dans la version BWV 532\
\bottomrule
\end{tabular}
\caption{Différences entre les fugues BWV 532 et BWV 532a relevées par Peter Williams (1980). Différences entre les fugues BWV 532 et BWV 532a relevées par Peter Williams (1980).}
\label{tab:fugue532an}
\end{largetable}
\end{fullwidth}
\end{document}
UPDATE II (long captions)
Using largetable with normal width and the new posted table

% !TeX TS-program = xelatex
\documentclass[a4paper,nobib,twoside,symmetric,justified,marginals=raggedouter, notoc]{tufte-book}
\makeatletter
\newlength{\oddshift}
\newcommand{\captionatbottom}{% from https://tex.stackexchange.com/a/229419/161015
\long\def@caption##1[##2]##3{%
\par
\addcontentsline{\csname ext@##1\endcsname}{##1}%
{\protect\numberline{\csname the##1\endcsname}{\ignorespaces ##2}}%
\begingroup
@parboxrestore%
\if@minipage@setminipage\fi%
\normalsize
@makecaption{\csname fnum@##1\endcsname}{\ignorespaces ##3}\par
\endgroup}
\long\def@makecaption##1##2{%
\vskip\abovecaptionskip%
@tufte@checkoddpage%
\ifthenelse{\boolean{@tufte@odd@page}}%
{\rlap{\parbox{\textwidth+\marginparwidth+\marginparsep}{\hskip0pt@tufte@caption@font##1: ##2}}}%
{\setlength{\oddshift}{-\marginparwidth-\marginparsep}\rlap{\hskip\oddshift\parbox{\textwidth+\marginparwidth+\marginparsep}{@tufte@caption@font##1: ##2}}}%
\vskip\belowcaptionskip%
}%
\let\caption@tufte@orig@caption%
\let\label@tufte@orig@label}
\newenvironment{largetable}{%
\@tufte@checkoddpage%
\ifthenelse{\boolean{@tufte@odd@page}}%
{\begin{table}[!htbp]\begin{adjustwidth}{}{}\captionatbottom}%
{\begin{table}[!htbp]\begin{adjustwidth}{-\marginparwidth-\marginparsep}{}\captionatbottom}}
{\end{adjustwidth}\end{table}}
\makeatother
\usepackage{booktabs}
\geometry{% normal pages
paperheight =297mm, paperwidth = 210mm, %layout=a4paper
layoutheight =240mm,layoutwidth =170mm,
textheight=38\baselineskip,
headheight=\baselineskip,
left=15mm, % right will be the same because of symmetric
bindingoffset= 5mm, % add space for binding
textwidth=90mm,
top=14mm,
marginparsep=6mm, marginparwidth=40mm,
includehead,
layoutvoffset= 28.5mm,layouthoffset= 20mm, % 297-240 = 57mm 210-170 = 40mm ->> to center smaller layout in a4
showframe=true,
showcrop=true }
\renewenvironment{fullwidth}
{\newgeometry{% full width pages
layoutheight =240mm,layoutwidth =170mm,
textheight=38\baselineskip,
headheight=\baselineskip,
left=15mm, % right will be the same because of symmetric
bindingoffset= 5mm, % add space for binding
textwidth=136mm, % 90+ 6+ 40
top=14mm,
marginparsep=0mm, marginparwidth=0mm, %changed to full width <<<<<<<<<<
includehead,
layoutvoffset= 28.5mm,layouthoffset= 20mm, % 297-240 = 57mm 210-170 = 40mm ->> to center smaller layout in a4
showframe=true,
showcrop=true }
\fancyhfoffset[LE]{0pt}
\fancyhfoffset[RO]{0pt}
}
{\restoregeometry}%
\usepackage{kantlipsum} % dummy text added
\RequirePackage{showframe} % margin lines <<<<<<<<<<<<<<<<<<<<<<<<<
\renewcommand\ShowFrameLinethickness{0.1pt}
\renewcommand\ShowFrameColor{\color{red}}
\newcommand{\textls}[2][5]{%
\begingroup\addfontfeatures{LetterSpace=#1}#2\endgroup
}
\renewcommand{\allcapsspacing}[1]{\textls[15]{#1}}
\renewcommand{\smallcapsspacing}[1]{\textls[10]{#1}}
\renewcommand{\allcaps}[1]{\textls[15]{\MakeTextUppercase{#1}}}
\renewcommand{\smallcaps}[1] {\smallcapsspacing{\scshape\MakeTextLowercase{#1}}}
\renewcommand{\textsc}[1]{\smallcapsspacing{\textsmallcaps{#1}}}
\usepackage{fontspec}
\usepackage{libertine} %<<<<<<<<<<<<<<<<
\setsansfont{TeX Gyre Heros}[Scale=MatchUppercase]
\usepackage{xspace}
\newcommand{\monthyear}{%
\ifcase\month\or janvier\or f'{e}vrier\or mars\or avril\or mai\or juin\or
juillet\or ao^{u}t\or septembre\or octobre\or novembre\or
d'{e}cembre\fi\space\number\year
}
\newcommand{\blankpage}{\newpage\hbox{}\thispagestyle{empty}\newpage}
\usepackage[english,french]{babel}
\frenchbsetup{StandardLayout=true}
\addto\captionsfrench{\def\tablename{Tableau}}
\addto\captionsfrench{\def\figurename{Figure}}
\frenchbsetup{IndentFirst=false}
\usepackage{csquotes}
\begin{document}
\frenchspacing
\mainmatter
\kant[9]
\begin{largetable}
\selectlanguage{french}
\footnotesize%
\begin{center}
\begin{tabular}{llll}
\toprule
\ & Manuels & Pédale & Compatibilité avec \ & & & l'\oe uvre BWV 532\
\midrule
Arnstadt & & \\
\quad Neue Kirche & CD-c$^3$ & CD-c$^1$d$^1$ & Non\\
Mühlausen & & \\
\quad Marienkirche & CD-c$^3$ & CD-d$^1$ & Oui \\
\quad Kirche Davi Blasii & CD-d$^3$ & CD-d$^1$ & Oui \\
\quad Brückenhofkirche & CD-c$^3$ & CD-c$^1$ & Non \\
& & \\
Weimar & & & \\
\quad Schlosskirche & CD-c$^3$ & CD-e$^1$ & Oui \\
\quad Jacobskirche & ? & ? & ? \\
\quad Stadtkirche & ? & ? & ? \\
& & \\
Halle & & & \\
\quad Marktkirche & CD-c$^3$ & CD-c$^1$ & Non \\
& & \\
C\oe then & & & \\
\quad Jacobskirche & CD-c$^3$ & CD-c$^1$ ou CD-d$^1$ & ? \\
\quad Agnuskirche & C-? & C-d$^1$e$^1$f$^1$ & ? \\
\quad Schlosskapelle & C-e$^3$ & C-e$^1$ & Oui \\
& & \\
Hambourg & & & \\
\quad Hauptkirche St Katharinen & CDEFGA-c$^3$ & CDE-d$^1$ & Oui \\
\quad Hauptkirche St Jacobi & CDE-C$^3$ (Rückpositif) & C-d$^1$e$^1$f$^1$ & Oui \\
& CDEFGA-c$^3$ (Werk) & & \\
& CDEFGA-c$^3$ (Oberpositiv) & & \\
& CDEFGA-c$^3$ (Brustpositiv) & & \\
\bottomrule
\end{tabular}
\end{center}
\caption{Tessiture des Manuels et Pédale des orgues joués par J.S. Bach entre 1703 et 1723, comparés à l'ambitus de l'\oe uvre (Manuels : D-b$^2$ ; Pédale : D-d$^1$).}
\label{tab:orgues}
%% \forceversofloat% forces caption to be set to the left of the float
\end{largetable}
\kant[11]
\begin{largetable}
\selectlanguage{french}
\footnotesize%
\begin{center}
\begin{tabular}{llll}
\toprule
\ & Manuels & Pédale & Compatibilité avec \ & & & l'\oe uvre BWV 532\
\midrule
Arnstadt & & \\
\quad Neue Kirche & CD-c$^3$ & CD-c$^1$d$^1$ & Non\\
Mühlausen & & \\
\quad Marienkirche & CD-c$^3$ & CD-d$^1$ & Oui \\
\quad Kirche Davi Blasii & CD-d$^3$ & CD-d$^1$ & Oui \\
\quad Brückenhofkirche & CD-c$^3$ & CD-c$^1$ & Non \\
& & \\
Weimar & & & \\
\quad Schlosskirche & CD-c$^3$ & CD-e$^1$ & Oui \\
\quad Jacobskirche & ? & ? & ? \\
\quad Stadtkirche & ? & ? & ? \\
& & \\
Halle & & & \\
\quad Marktkirche & CD-c$^3$ & CD-c$^1$ & Non \\
& & \\
C\oe then & & & \\
\quad Jacobskirche & CD-c$^3$ & CD-c$^1$ ou CD-d$^1$ & ? \\
\quad Agnuskirche & C-? & C-d$^1$e$^1$f$^1$ & ? \\
\quad Schlosskapelle & C-e$^3$ & C-e$^1$ & Oui \\
& & \\
Hambourg & & & \\
\quad Hauptkirche St Katharinen & CDEFGA-c$^3$ & CDE-d$^1$ & Oui \\
\quad Hauptkirche St Jacobi & CDE-C$^3$ (Rückpositif) & C-d$^1$e$^1$f$^1$ & Oui \\
& CDEFGA-c$^3$ (Werk) & & \\
& CDEFGA-c$^3$ (Oberpositiv) & & \\
& CDEFGA-c$^3$ (Brustpositiv) & & \\
\bottomrule
\end{tabular}
\end{center}
\caption{Tessiture des Manuels et Pédale des orgues joués par J.S. Bach entre 1703 et 1723, comparés à l'ambitus de l'\oe uvre (Manuels : D-b$^2$ ; Pédale : D-d$^1$).}
\label{tab:orgues}
%% \forceversofloat% forces caption to be set to the left of the float
\end{largetable}
\end{document}
UPDATE III (long captions in the margin)
Using largetable with normal width and the new posted table

% !TeX TS-program = xelatex
\documentclass[a4paper,nobib,twoside,symmetric,justified,marginals=raggedouter, notoc]{tufte-book}
\makeatletter
\newlength{\oddshift}
\newcommand{\captionatbottommargin}{% from https://tex.stackexchange.com/a/229419/161015
\long\def@caption##1[##2]##3{%
\par
\addcontentsline{\csname ext@##1\endcsname}{##1}%
{\protect\numberline{\csname the##1\endcsname}{\ignorespaces ##2}}%
\begingroup
@parboxrestore%
\if@minipage@setminipage\fi%
\normalsize
@makecaption{\csname fnum@##1\endcsname}{\ignorespaces ##3}\par
\endgroup}
\long\def@makecaption##1##2{%
\vskip\abovecaptionskip%
@tufte@checkoddpage%
\ifthenelse{\boolean{@tufte@odd@page}}%
{\setlength{\oddshift}{\textwidth}\rlap{\hskip\oddshift\parbox{\marginparwidth+\marginparsep}{\hskip0pt@tufte@caption@font##1: ##2}}}%
{\setlength{\oddshift}{-\marginparwidth-\marginparsep}\rlap{\hskip\oddshift\parbox{\marginparwidth+\marginparsep}{@tufte@caption@font##1: ##2}}}%
\vskip\belowcaptionskip%
}%
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\begin{document}
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\mainmatter
9. \kant[9]
\begin{largetable}
\selectlanguage{french}
\footnotesize%
\begin{tabular}{llll}
\toprule
\ & Manuels & Pédale & Compatibilité avec \ & & & l'\oe uvre BWV 532\
\midrule
Arnstadt & & \\
\quad Neue Kirche & CD-c$^3$ & CD-c$^1$d$^1$ & Non\\
Mühlausen & & \\
\quad Marienkirche & CD-c$^3$ & CD-d$^1$ & Oui \\
\quad Kirche Davi Blasii & CD-d$^3$ & CD-d$^1$ & Oui \\
\quad Brückenhofkirche & CD-c$^3$ & CD-c$^1$ & Non \\
& & \\
Weimar & & & \\
\quad Schlosskirche & CD-c$^3$ & CD-e$^1$ & Oui \\
\quad Jacobskirche & ? & ? & ? \\
\quad Stadtkirche & ? & ? & ? \\
& & \\
Halle & & & \\
\quad Marktkirche & CD-c$^3$ & CD-c$^1$ & Non \\
& & \\
C\oe then & & & \\
\quad Jacobskirche & CD-c$^3$ & CD-c$^1$ ou CD-d$^1$ & ? \\
\quad Agnuskirche & C-? & C-d$^1$e$^1$f$^1$ & ? \\
\quad Schlosskapelle & C-e$^3$ & C-e$^1$ & Oui \\
& & \\
Hambourg & & & \\
\quad Hauptkirche St Katharinen & CDEFGA-c$^3$ & CDE-d$^1$ & Oui \\
\quad Hauptkirche St Jacobi & CDE-C$^3$ (Rückpositif) & C-d$^1$e$^1$f$^1$ & Oui \\
& CDEFGA-c$^3$ (Werk) & & \\
& CDEFGA-c$^3$ (Oberpositiv) & & \\
& CDEFGA-c$^3$ (Brustpositiv) & & \\
\bottomrule
\end{tabular}
\caption{Tessiture des Manuels et Pédale des orgues joués par J.S. Bach entre 1703 et 1723, comparés à l'ambitus de l'\oe uvre (Manuels : D-b$^2$ ; Pédale : D-d$^1$).}
\label{tab:orgues1}
%% \forceversofloat% forces caption to be set to the left of the float
\end{largetable}
\kant[11]
\begin{largetable}
\selectlanguage{french}
\footnotesize%
\begin{tabular}{llll}
\toprule
\ & Manuels & Pédale & Compatibilité avec \ & & & l'\oe uvre BWV 532\
\midrule
Arnstadt & & \\
\quad Neue Kirche & CD-c$^3$ & CD-c$^1$d$^1$ & Non\\
Mühlausen & & \\
\quad Marienkirche & CD-c$^3$ & CD-d$^1$ & Oui \\
\quad Kirche Davi Blasii & CD-d$^3$ & CD-d$^1$ & Oui \\
\quad Brückenhofkirche & CD-c$^3$ & CD-c$^1$ & Non \\
& & \\
Weimar & & & \\
\quad Schlosskirche & CD-c$^3$ & CD-e$^1$ & Oui \\
\quad Jacobskirche & ? & ? & ? \\
\quad Stadtkirche & ? & ? & ? \\
& & \\
Halle & & & \\
\quad Marktkirche & CD-c$^3$ & CD-c$^1$ & Non \\
& & \\
C\oe then & & & \\
\quad Jacobskirche & CD-c$^3$ & CD-c$^1$ ou CD-d$^1$ & ? \\
\quad Agnuskirche & C-? & C-d$^1$e$^1$f$^1$ & ? \\
\quad Schlosskapelle & C-e$^3$ & C-e$^1$ & Oui \\
& & \\
Hambourg & & & \\
\quad Hauptkirche St Katharinen & CDEFGA-c$^3$ & CDE-d$^1$ & Oui \\
\quad Hauptkirche St Jacobi & CDE-C$^3$ (Rückpositif) & C-d$^1$e$^1$f$^1$ & Oui \\
& CDEFGA-c$^3$ (Werk) & & \\
& CDEFGA-c$^3$ (Oberpositiv) & & \\
& CDEFGA-c$^3$ (Brustpositiv) & & \\
\bottomrule
\end{tabular}
\caption{Tessiture des Manuels et Pédale des orgues joués par J.S. Bach entre 1703 et 1723, comparés à l'ambitus de l'\oe uvre (Manuels : D-b$^2$ ; Pédale : D-d$^1$).}
\label{tab:orgues2}
%% \forceversofloat% forces caption to be set to the left of the float
\end{largetable}
\end{document}