List of Good books in 2011 spring:

1. Drupal’s Building Blocks: Quickly Building Web Sites with CCK, Views, and Panels

Most basic elements in building a website with Drupal. Config/backend stuff. A good start for Drupal developer.

2. Drupal 6 Themes

Frontend stuff, i.e., how drupal use html templates and the integration in between.

3. Using Drupal

advanced version of book1. More building elements in Drupal.

4. Pro Drupal 6 Development 2nd ed

Pure programming stuff. For customization that specific to special functions that no one did before. API, hooks, behind the scenes.