Or, how to stop the new page if a new chapter starts?
The real question is why there is allways a new page, if chapter is used?
If you take a look on book.cls you will find this:
\newcommand\chapter{\if@openright\cleardoublepage\else\clearpage\fi
\thispagestyle{plain}%
\global\@topnum\z@
\@afterindentfalse
\secdef\@chapter\@schapter}
to stop the new page clearpage and cleardoublepage must be deleted and chapter command has to re-newed in a way like this:
\makeatletter
\renewcommand\chapter{\thispagestyle{plain}%
\global\@topnum\z@
\@afterindentfalse
\secdef\@chapter\@schapter}
\makeatother