drupalib A place for library drupallers to hang out

Learning Drupal 6 Module Development by Matt Butcher is a useful counterpart to Pro Drupal Development (PDD) in two ways -- it updates some of the content in PDD (which covers Drupal 5.x) and it focuses on some topics not covered in sufficient detail in the earlier, and intentionally more general, book, such as AJAX/JSON, in-module theming, and installation profiles.

Pro Drupal Development, by John VanDyk and Matt Westgate (Apress, 2007) is the first book aimed specifically at Drupal developers.