latex no new page in chapter

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


latex book font size 14 pt

How to get font size 14 pt by using book as documentclass?

If you are using the default size i.e. 10 pt, the command \Large will change the size up to 14 pt.

\documentclass{book}
\begin{document}
\Large
some text
\end{document}


This also will work, if you set normalsize to 11 pt:

\documentclass[11pt]{book}
\begin{document}
\Large
some text
\end{document}


But if you set normalsize to 12 pt then use the command \large:

\documentclass[12pt]{book}
\begin{document}
\large
some text
\end{document}