Pro Drupal Development, Second Edition has been released for Drupal 6!
While I haven't gone through the second edition yet, this book written for Drupal has been an incredible resource. It is very well organized, easy to follow along with, and has plenty of code samples to help you on your way.
Once you've read it, it becomes an excellent reference to go back to. I've had my copy for a while and although I reference it a bit less than before, it still doesn't seem to stay put on my bookshelf like most other books.
I heard rumors that there was not going to be a version 6, but I'm happy they went ahead with it.
Here's a snippet from the publisher: