I have been working on a couple of projects using Drupal lately that required creating some custom modules. So again, I flipped thru my well-worn copy of Pro Drupal Development. Here is my earlier review of Pro Drupal Development.
Drupal has released version 6 now. So I click to the Apress site and lo and behold, they're already working on Pro Drupal Development, 2nd edition which will feature updates for Drupal 6. If it is as good as the first book, it will be a must have for any Drupal developer.