While it cannot possibly cover everything for the Drupal 7 API, this is a solid book for getting the background to write custom code and/or contributed Drupal 7 modules.
Why does the world work the way it does? A question I've been asking myself for a long time. I finally realized it has a lot to do with hook_form_alter();
I got a job doing Drupal development. I read this book over the weekend to prepare for my first day. By Monday, I knew how to use Drupal better than anyone at the office. Great book.
Very good book. Explained Drupal 7 nicely to build the conceptual understanding of Drupal CMS. Still I refer This book whenever in dilemma or any associated topics comes up