Q: photos hang when generating book - why?

I'm having the same problem, how did you get through this?

First preview the book according to this Apple document and save the pdf file: Preview a print project in Photos, iPhoto, or Aperture to avoid issues - Apple Support.  If that's successful the boot into Safe Mode, Mac OS X: Starting up in Safe Mode, and try ordering from there.

Compare the printed copy with the PDF file when it arrives.