I need a single page (page 3 in my example) to be shorter than the rest of the pages. I don't know where the page break is (3 to 4), so I can't manually insert \restoregeometry. Therefore I've tried with the package (command) afterpage, but this has no visible effect.
Pages 1 to 3 are fine, pages 4 and later should look similar to first two pages.
\documentclass{book}
\usepackage{geometry,blindtext,afterpage}
\geometry{textheight=183.5mm}
\begin{document}
\blindtext[10]
\newgeometry{textheight=50mm}
\noindent This is a special (short) page
\afterpage{\restoregeometry}
\blindtext[10]
\end{document}
There is this closed question which is very similar to mine.