I would like to typeset a book where the odd/even pages have the same format: each has a long text section on the left and a place to put small marginal notes on the right. If necessary, I want the environments for tables/code in the text to be able to expand into this right margin.
But, I would like to get the standard odd/even page headers for chapters/sections.
The KOMA-Script classes offer the class option twoside=semi:
Left/right margins and positioning of marginal notes match that of twoside=false (oneside for the standard classes);
Header/footer content/positioning match that of twoside=true (twoside for the standard classes).
\documentclass[twoside=semi]{scrbook}
\usepackage{lipsum}
\begin{document}
\chapter{foo}
\section{foobar}
Some text.\marginpar{First marginal note} \lipsum[1-6]
Some text.\marginpar{Second marginal note} \lipsum[7-12]
\end{document}