Pro Drupal 7 Development, Third Edition by Dave Reid
My rating: 2 of 5 stars
Decent book on Drupal development. It helped me gain understanding on how Drupal works. It does suffer two flaws: There are quite a few typos, and this can be frustrating especially when they are in code. Drupal is a moving target, so even though it is Drupal 7, some of the exercises seem to be developed around Drupal 6 or an very early release of 7. When using this book make sure to check the errata on the apress website.