How to prevent book class from vertically justifying the contents?

Question

If we use the book class, the output looks like the following screenshot.

But if we use the article class, the output looks better as follows.

The MWE is given as follows. Note: please enable -shell-escape!

\documentclass[12pt]{article}% please change to article and compare the output.

\usepackage{filecontents}
\begin{filecontents*}{"article sample.tex"}
\documentclass{article}
\usepackage{lipsum}
\begin{document}
\lipsum[1-20]
\end{document}
\end{filecontents*}

\usepackage{showexpl}

\lstset
{
    breaklines=true,
    breakindent=0pt,
    tabsize=2,
    columns=flexible,
    language={[LaTeX]TeX},
    basicstyle=\small\ttfamily\hbox{},
}

\newcount\x

\newcommand\MyLTXinputExample[2][2mm]{%
% #1: length of gap,
% #2: filename with .tex, 
    \lstinputlisting[caption=\url{#2.tex}]{"#2.tex"}
    \bgroup
        \parskip=#1\relax
        \fboxsep=0pt
        \pdfximage{#2.pdf}%
        \x=0
        \loop
            \ifnum\x<\pdflastximagepages
                \advance\x by 1
                \noindent\hfill
                \fbox{\includegraphics[width=0.5\dimexpr\linewidth-4\fboxrule-#1\relax,page=\x]{"#2"}}%                     
            \ifnum\x<\pdflastximagepages
                \advance\x by 1
                \hfill
                \fbox{\includegraphics[width=0.5\dimexpr\linewidth-4\fboxrule-#1\relax,page=\x]{"#2"}}%
            \fi
                \hfill\null\par
        \repeat
    \egroup
}

\PassOptionsToPackage{obeyspaces}{url}
\usepackage{hyperref}

\begin{document}
\immediate\write18{pdflatex "article sample"}
\MyLTXinputExample{article sample}
\end{document}

How to prevent book class from vertically justifying the contents? In other words, I want to use the book class but its output (only the justification is under consideration) must look like the output that article class produces.

Answer 1

Put a \raggedbottom into your preamble.