Pro Drupal Development
When I met John and Matt, back at DrupalCon Sunnyvale in March, they were still in the final stages of writing Pro Drupal Development. But now it's out. And there's no doubt at all: this is the book on Drupal. It's so good, it even scored a Slashdot review (and that's no small feat). This book really does have everything the professional Drupal developer needs to know. Pereonally, I like the final chapters most (on security and performance), and I'm confident that they'll be a reference asset to me for years to come. Got the book today in Boston, as it will undoubtedly be more expensive back home than it is here in the USA.