Over the past year I've managed to build up a good collection of Drupal books, but haven't had time to read many of them yet:

• Cracking Drupal - read a chapter or two, need to read it in detail and take copious notes, this is very important book that all developers should read.
• Drupal Performance Tips - arrives tomorrow (!), will be putting this along with Cracking Drupal at the top of my must-read-NAAAAOOOW!! list.
• Pro Drupal Development (Drupal 6 edition) - started reading it in November 2008, got about 1/3rd of the way through, need to start over and take proper notes this time.
• Drupal 6 Search Engine Optimization - looked at a chapter or two, seems to be good info though lots of basics, need to finish reading it.
• Drupal 6 Content Administration - aimed at the end-user rather than the developer, need to finish skimming through it.
• Using Drupal - read a few chapters, need to finish reading it, had a few ideas that were new to me.
• Drupal Multimedia - skimmed through the entire book, lots of basic information on site building, only a little detail I wasn't already aware of.