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}