Less than two years ago, I wrote the foreword for the first edition of Pro Drupal Development book. What was missing at that time was a developer book for Drupal. By writing the first version of this book, John and Matt made an incredible contribution to Drupal's steady growth. The book cracked the Amazon Top 100 and was one of my personal highlights of 2007. More than 10,000 copies were sold! It still blows my mind to think that one book has given the world more than 10,000 new or greatly improved Drupal developers.
Probably to John and Matt's despair (sorry!), all of the chapters of the original Pro Drupal Development book went partially out of date. Fortunately, the second edition of this book fixes all that and it started shipping a couple of weeks ago! Check it out at http://drupalbook.com.
The second edition of Pro Drupal Development covers all of the capabilities and developer facilities in Drupal 6, and provides deep insight into the inner workings and design choices behind Drupal 6. If anything was missing for Drupal 6, it was this book, and I'm already indebted to John for revising and expanding it.