Fill page with text

Question

With command \raggedbottom lines on pages are aligned. I need it, \flushbottom generate quite ugly book spread, but in result some pages have different height of text block. I tried play agound with "glue" \spaceskip=\fontdimen2\font plus 5\fontdimen2\font minus 0.3pt and \emergencystretch = 2em, but it don't have an effect on all book pages. Is there any good solution, or i need to check every page and play with glue there directly? \defaultfontfeatures{Microtypography,Ligatures={TeX},Renderer=Basic} have no effect too. MWE is:

\documentclass{book}
\usepackage{polyglossia}
\usepackage[hyphenation,parindent,lastparline,nosingleletter]{impnattypo}
\usepackage[defaultlines=2,all]{nowidow}
\usepackage{lipsum}
\usepackage[
    paperheight=215mm,
    paperwidth=145mm,
    top=2cm, 
    bottom=2.5cm,
    inner=2cm,
    outer=2cm
]{geometry}
\setmainlanguage{english}
\defaultfontfeatures{Microtypography,Ligatures={TeX},Renderer=Basic}
\protrudechars=2 % or \pdfprotrudechars=2 and
\adjustspacing=2 %    \pdfadjustspacing=2 with luatex < v0.85
\newfontfeature{Microtypography}{protrusion=default;expansion=default}
\directlua{fonts.protrusions.setups.default.factor=1}
\setmainfont{Arial}
\frenchspacing
\raggedbottom
\begin{document}
\lipsum[1-150]
\end{document}

Answer 1

The marked white space (or at least equivalent space in the test document) at the bottom of the page was occuring as you were explicitly allowing it via the \raggedbottom declaration, and then forcing the break there by disallowing one line of the next paragraph to be placed on that page.

Using \flushbottom tells tex not to allow space in that position

Soecifying heightrounded to geometry ensures that it is possible to fit in continuos text and achieve the required bottom baseline, by forcing the text height is an integer multiple of \baselineskip plus \topskip.

Replacing the no-widow package by widows-and-orphans allows the pages to be filled but warns of widows and orphans in the log:

Package widows-and-orphans Warning: Orphan on page 6
Package widows-and-orphans Warning: Widow on page 13
Package widows-and-orphans Warning: Orphan on page 17
Package widows-and-orphans Warning: Widow on page 23
Package widows-and-orphans Warning: Orphan on page 31
Package widows-and-orphans Warning: Orphan on page 32

You can then adjust each in turn re-wording or using \enlargethispage or here I just used \looseness to force some paragraphs to take a line less or a line more than they would otherwise, resulting in 36 pages all full with no widows or orphans

\documentclass{book}
\usepackage{polyglossia}
\usepackage[hyphenation,parindent,lastparline,nosingleletter]{impnattypo}
%\usepackage[defaultlines=2,all]{nowidow}
\usepackage{widows-and-orphans}
\usepackage{lipsum}
\usepackage[
    paperheight=215mm,
    paperwidth=145mm,
    top=2cm, 
    bottom=2.5cm,
    inner=2cm,
    outer=2cm,
    heightrounded
]{geometry}
\setmainlanguage{english}
\defaultfontfeatures{Microtypography,Ligatures={TeX},Renderer=Basic}
\protrudechars=2 % or \pdfprotrudechars=2 and
\adjustspacing=2 %    \pdfadjustspacing=2 with luatex < v0.85
\newfontfeature{Microtypography}{protrusion=default;expansion=default}
\directlua{fonts.protrusions.setups.default.factor=1}
\setmainfont{Arial}
\frenchspacing
\flushbottom
\begin{document}
% just so I can see which para is which
\everypar{\stepcounter{enumi}\makebox[0pt][r]{\theenumi\ }}
\lipsum[1-24]
\looseness=1
\lipsum[25]
\lipsum[26-32]
\looseness=1
\lipsum[33]
\lipsum[34-71]
\looseness=1
\lipsum[72]
\lipsum[73-77]
\looseness=1
\lipsum[78]
\lipsum[79-80]
\looseness=1
\lipsum[81]
\lipsum[82]
\lipsum[83]
\looseness=1
\lipsum[84]
\lipsum[85-108]
\looseness=-1
\lipsum[109]
\lipsum[110-128]
\looseness=1
\lipsum[129]
\lipsum[130-150]
\end{document}