Ok, I have followup, outside the group, I assume the value of \dime0 can be anything? Used by Latex as scratch? When I do this:
\documentclass[12pt]{book}%
\begin{document}
\the\textwidth\par
\the\dimexpr0.9\textwidth\relax\par
\begingroup\dimen0=0.9\textwidth\the\dimen0\endgroup
\the\dimen0
\end{document}
I see this