It is a strange title for a series of books, I know, but these two books I recommend highly. While they are somewhat technical, the author (Thord Daniel Hedengren) has a pleasant style and the books are ideal for the self learner.
The first book focuses on theme design and the examples of theme design included are worked through step by step from concept to final design in the book.
In the second book the emphasis is more on understanding the technology of WordPress and how it can be used for applications other than blogging.
Both books are full-colour, high quality. They are a pleasure to own and read.
- Smashing WordPress Themes: Making WordPress Beautiful (Smashing Magazine Book Series)
- Smashing WordPress: Beyond the Blog (Smashing Magazine Book Series)