There is an absolutely awesome book (and e-book) called "Digging Into WordPress". Highly recommended. Solid advice, excellent descriptions and good code. They're really quite good at explaining things. If you buy a physical book you also get the PDF. The other nice thing is that if you buy it you get future updates as well, so that when WP goes through changes, you get the new versions of the PDF as it's updated.
You can find it at http://digwp.com/
