\documentclass{book}
\usepackage{adjmulticol,lipsum}
\evensidemargin=1.5in
\oddsidemargin=-3em
\textwidth=5in
\begin{document}

\textbf{main body margins, as defined.}
\marginpar{[marginpar]}
\lipsum[1-2]
 \begin{adjmulticols}{1}{12pt}{-2in}
   \textbf{multicols, 1, 12pt, -2in}
  \lipsum[3-4]
\end{adjmulticols}
\textbf{back main text}
\marginpar{[marginpar]}
\lipsum[12-14]
\marginpar{[marginpar]}
 \begin{adjmulticols}{2}{12pt}{-2in}
   \textbf{multicols, 2, 12pt, -2in}
  \lipsum[6-15]
\end{adjmulticols}
\textbf{back main text}
\marginpar{[marginpar]}
\lipsum
\marginpar{[marginpar]}
 \begin{adjmulticols*}{2}{12pt}{-2in}
   \textbf{multicols, 2, 12pt, -2in}
  \lipsum[6-16]
\end{adjmulticols*}
\end{document}
