Latex is capable of having a shortened header for the long chapter title using \chaptermark{<shortened title>}, which is similar to using the optional argument to \chapter[<ToC entry>]{<title>} to display a different ToC entry to what is displayed in the main document.
Whilst there exists a similar thing for \section, it seems that titles for \subsection and \subsubsection are not displayed in the page headers.
It has be mentioned that it is rare for documents to use titles below \section in a running head. Is this a design decision of Latex ?
\documentclass[12pt]{book}
\usepackage{lipsum}
\usepackage{geometry}
%% 13x21 cm Page Size
\geometry{ paperwidth=13cm, paperheight=21cm,
%textwidth=12cm, textheight=20cm, % This is not needed
margin=8mm, includehead}
\begin{document}
\tableofcontents
\chapter{This is an introduction to some topic}
\chaptermark{Introduction}
\section[Section Heading]{This is a section discussion about this and that}
\lipsum[1-30]
\subsection[Subsection Heading]{This is a subsection discussion about here and there}
\lipsum[1-30]
\subsubsection[Subsubsection Heading]{This is a subsubsection discussion about wear and tear}
\lipsum[1-30]
\end{document}
\sectionmarkfor\section,\subsectionmarkfor\subsectionetc. And some classes like the KOMA-Script class or memoir also provide extended optional arguments for\chapteretc. to distinguish between ToC entry and page header. – cabohah Oct 14 '23 at 14:20\sectionin a running head. – barbara beeton Oct 14 '23 at 18:06