Looks like they are still ironing out the issues with the book module in Moodle 2:-
It will probably be stable by the summer, so I guess that it all depends on when you want to move to Moodle 2...
Thanks for your reply and the link. I couldn't find much indication of timescales on the Moodle site, but summer would be fine for a stable book module. I don't think my client is rushing into upgrading. I didn't relish the thought of turning my books into lessons.