Professional WordPress: Design and Development
Brad Williams. David Damstra and Hal Stern
Professional WordPress Plugin Development
Brad Williams, Ozh Richard, Justin Tadlock
These are two highly recommended books on WordPress. They are highly technical and you require an in-depth knowledge of MySQL, PHP programming, HTML and CSS.
That said, these books really take you “under the hood” as it were to describe the inner workings of WordPress. The provide a lot of example code and you can download the code from the book website.
The books are comprehensive but not easy to read.