Drupal 6: Ultimate Community Site Guide
Using Drupal
Pro Drupal 7 Development (Expert's Voice in Open Source)
Drupal For Dummies (For Dummies (Computers))
Building Powerful and Robust Websites with Drupal 6: Build your own professional blog, forum, portal or community website with Drupal 6
Drupal 6 Themes: Create new themes for your Drupal 6 site with clean layout and powerful CSS styling
Cracking Drupal: A Drop in the Bucket
Drupal E-commerce with Ubercart 2.x
Front End Drupal: Designing, Theming, Scripting (Developer's Library)