I’ve been in the process of deciding whether or not to move atlantic.edu over to Drupal. While the online documentation and the various online sources of info are pretty solid, I really was looking for a concise resource that is aimed more towards the intermediate or advanced PHP developer.
Pro Dupal Development Second Edition has really been a great read for what I was looking for. It does an excellent job of breaking down the various modules, hooks, themes, nodes, blocks, etc. of Drupal. Drupal is very powerful, but at times I find it’s learning curve can be a bit on the steep side. This book really does an excellent job of explaining things from a developers perspective.
The Second Edition covers Drupal 6, which is what I will most likely initially be running. There is a Third Edition due out around this time that will cover Drupal 7 if that’s of more interest (and something I’ll probably end up getting eventually too).