If you are slightly interested on diving into the WordPress API either to make better themes or to craft some awesome plugins then you really have to check out Professional WordPress Plugin Development book. This piece of gold summarizes and explores each part of the WordPress API so cleverly that it makes it a must have guide for any WordPress Developer out there.
The well known authors ( Brad Williams, Ozh Richard, Justin Tadlock ) have made a terrific job on making the content engaging and easy to follow. Think of it as a sum of Codex/API Guide + Examples + Explanations of those examples + Best Practices + Even more!! So whether you are a seasoned WordPress Developer or just starting with it, I strongly recommend you to grab a copy of this book.
Here’s a little video from Brad Williams introducing the book.