> It's a 100 page paperback book, with illustrations and text, on facing pages. The pages are yellowed, so it's not an option simply scan to PDF - they'll need to be cleaned up.

If is a repetitive task it looks more like a job to automatize on imagemagick. Gimp can do it also with plugins, but I would meditate first about imagemagick instead to repeat the same sequence of commands 100 times on Gimp.

Then use a quick script on Perl or your preferred language of choice for writing the main part of the tex file for you. Something like this on pseudocode:

open file output

print $output "lines of preamble";

for each $file in directory "pagesofbook"

{print output.tex "\begin{figure}\includegraphics{$file}\end{figure};\n;}

print output.tex "\end{document}";

close output.tex;

then fine-tune the texfile