marginnote always on left side of the page

As there are tests for twoside everywhere in the code (and it affects spacing and other things), it is imho the best to locally set twoside to false:

\documentclass[twoside]{book}
\usepackage[centering,marginparwidth=2.5cm,paperheight=10cm]{geometry}
\usepackage{marginnote}
\usepackage{lipsum}
\usepackage{etoolbox}

\makeatletter
\patchcmd{\@mn@@@marginnote}{\begingroup}{\begingroup\@twosidefalse}{}{\fail}
\reversemarginpar
\makeatother

\begin{document}

\lipsum[1]\marginnote{test test test test test}
\lipsum[1-2]\marginnote{test test test test test}
\lipsum[1-2]\marginnote{test test test test test}
\lipsum[1-2]\marginnote{test test test test test}
\lipsum[1-2]\marginnote{test test test test test}
\lipsum[1-2]

\end{document}