%********a.tex
\documentclass{book}
\usepackage[final]{pdfpages}
\begin{document}
Hello
\includepdf[pages=-]{b.pdf}
\end{document}
Now b.tex
%%%%%%%%%%
%********b.tex
\documentclass{book}
\begin{document}
World!
\end{document}
%%%%%%%%%%%
%********a.tex
\documentclass{book}
\usepackage[final]{pdfpages}
\begin{document}
Hello
\includepdf[pages=-]{b.pdf}
\end{document}
Now b.tex
%%%%%%%%%%
%********b.tex
\documentclass{book}
\begin{document}
World!
\end{document}
%%%%%%%%%%%
Perhaps, this works as a 'kickstart' for the OP...
Change the title, styles at will ...
\documentclass{book}
\renewcommand{\partname}{Book}
% If chapters should be reset when a new part ('book') starts then
% use this code
\makeatletter
\@addtoreset{chapter}{part}
\makeatother
\begin{document}
\tableofcontents
\part{This is the 1st book}
\chapter{The first chapter of the first part}
\part{This is the 2nd book}
\chapter{The first chapter of the 2nd part}
\chapter{The second chapter of the 2nd part}
\part{Book 3}
\chapter{The first chapter of the 3rd part}
\chapter{The second chapter of the 3rd part}
\part{Book 4}
\end{document}

I never used the syntax \includepdf[<option>]{file.pdf} so I probably ignore its advantages.
I specify the following because you mentioned in the title that you own tex sources of your files a.pdf and b.pdf.
Just editing the template from the previous answer by @Christian, I would suggest you use the command \include{}:
\begin{document}
\part{This is the 1st book}
\chapter{The first chapter of the first part}
\include{a}%% INCLUDE filename
\part{This is the 2nd book}
\chapter{The first chapter of the 2nd part}
\include{b1}%% INCLUDE filename
\chapter{The second chapter of the 2nd part}
\include{b2}%% INCLUDE filename
\end{document}
or the command \input{}:
\begin{document}
\tableofcontents
\part{This is the 1st book}
\chapter{The first chapter of the first part}
\input{a.tex}%% INPUT filename.tex
\part{This is the 2nd book}
\chapter{The first chapter of the 2nd part}
\input{b1.tex}%% INPUT filename.tex
\chapter{The second chapter of the 2nd part}
\input{b2.tex}%% INPUT filename.tex
\end{document}
to do the task. Differences between input and include here.
\part{Book 1} ... \part{Book 4}command? You can split yourTeXcode however to several.texfiles and include them afterwards in any order you desire. – Apr 22 '14 at 08:16