Twice now I've had the Book hierarchy on a Drupal 6 site go bonkers. The first time was on this very site, back in 2009, while this site was being converted from Drupal 5 to Drupal 6 (it's now on Drupal 7). The /book page simply became empty, and I managed to fix the book hierarchy with some database twiddling. That was over six years ago and I'd since converted this site to Drupal 7. I have another site still running Drupal 6, and the same problem (empty /book page) just happened on that site.