Decimo tertio Kalendas Apriles MMXIII scripsit Thomas Morley :
in the code below you'll see increasing page-numbers throughout the
book as usual.
Ooh, now that I have had the time to read your code, I see what you
did, and I like it. The \on-the-fly side-effect is devious (and
un-Scheme-like, but who cares). Thanks!
Though, I didn't manage to code "(1/3)" for consecutive bookparts,
because I didn't understand how to get or calculate the
bookpart-last-page-number.
The simplest way to do it would be the TeX way: saving the page count
for each section in a .aux file and reading this again on second
compilation. Unless, of course, the (open) Scheme function is disabled
in Lilypond. I will have a try at this.