Ah... I had tried this before, failed, and earlier today, gave it another shot, which created another empty book. Then this evening, what I think was the same setup happily generated a book. The FAQ says
"Why can't I generate a LJBook even if my login/password and UTF-8 conversions are OK ? LiveJournal will not allow more than 3 backups per hour. Thus LJBook's requests will be rejected. Allow 2-3 hours and retry."
I think that means 3 backups overall for LJBook, which means there's a reasonably good chance if you put in the same request a while later -- it might work. As long as not too many people are trying to use it at the same time!
Re: LJ-Book
"Why can't I generate a LJBook even if my login/password and UTF-8 conversions are OK ?
LiveJournal will not allow more than 3 backups per hour. Thus LJBook's requests will be rejected.
Allow 2-3 hours and retry."
I think that means 3 backups overall for LJBook, which means there's a reasonably good chance if you put in the same request a while later -- it might work. As long as not too many people are trying to use it at the same time!
Give it another shot, and see if it works.