% Imposition of crito-book.pdf in one signature: two A5 pages on each side of
% a landscape A4 sheet, in the right order to fold in half and sew.
% Print on both sides, flipping on the short edge.
% Compile crito-book.tex first.
\documentclass{article}
\usepackage[a4paper,landscape]{geometry}
\usepackage{pdfpages}
\usepackage{iftex}
\def\bookpdf{crito-book.pdf}
\newcount\N \newcount\M \newcount\H \newcount\i \newcount\a \newcount\b
\ifPDFTeX \pdfximage{\bookpdf}\N=\pdflastximagepages
\else\ifXeTeX \N=\XeTeXpdfpagecount"\bookpdf"\relax
\else \saveimageresource{\bookpdf}\N=\lastsavedimageresourcepages
\fi\fi
\M=\N \advance\M 3 \divide\M 4 \multiply\M 4 % a multiple of 4
\H=\M \divide\H 2
% order: M,1 | 2,M-1 | M-2,3 | 4,M-3 ... (page > N = blank)
\def\pg#1{\ifnum#1>\N {}\else #1\fi}
\def\order{}
\i=0
\loop\ifnum\i<\H
  \a=\M \advance\a-\i \b=\i \advance\b 1
  \edef\order{\order\pg{\the\a},\pg{\the\b},}
  \advance\b 1 \advance\a-1
  \edef\order{\order\pg{\the\b},\pg{\the\a},}
  \advance\i 2
\repeat
\def\dropcomma#1,\dropend{#1}\edef\order{\expandafter\dropcomma\order\dropend}% the final comma
\begin{document}
\edef\x{\noexpand\includepdf[pages={\order},nup=2x1]{\bookpdf}}\x
\end{document}