\documentclass{book}
\usepackage{lipsum,multicol}
\usepackage{adjmulticol}
\evensidemargin=1.5in
\oddsidemargin=-3em
\textwidth=5in
\newenvironment{chmargin}[2]{%
\begin{list}{}{\topsep0pt\partopsep0pt\itemsep0pt\parsep\parskip%
\listparindent\parindent\itemindent\parindent
\leftmargin#1\rightmargin#2\relax
%
\leftmargin#2\rightmargin#1\relax
}\item}%
{\end{list}}
\begin{document}
\chapter{Show}
\section{Normal}
\marginpar{I need a lot of outer margin space in the normal text.}
\lipsum[1-10]
\begin{chmargin}{0pt}{-160pt}
\section{End of Chapter Meterial}
\textbf{Now I want to use the outer margin because I need width.}
\lipsum[11-16]
\fbox{\textbf{\Large Darn---it extends into the inner spine margin now.}}
\lipsum[17-40]
\end{chmargin}
\end{document}