Drupal 7 Module Development
From John Albin Wilkins, author of Chapters 3 & 4: Only a handful of people claim to understand the entire Drupal theming system, so I was very happy to share my experience with theming strategies and my knowledge of the theme system internals. These chapters represent the first thorough explanation of Drupal 7's theme system, a critical system all module developers will need to interact with.