This is a really comprehensive resource for all aspects of Wordpress customisation, with extensive information on advanced template development, and LOTS of explanation on how the various page elements work. There are countless PHP examples given which you can use straight away in your site, although if you're not au fait with PHP, the author provides plenty of explanatory notes to go along with them. The sections on producing widgets and plugins is particularly clear and thorough.
You can really go into this completely 'cold' in terms of Wordpress development - there are some fantastic chapters on the basic under-the-hood workings of Wordpress, particularly 'The Loop'. It's definitely the most accessible introduction / explanation of this that I've come across, and it's really helped me to 'get' Wordpress.
What's nice is that it's not written with a purely technical focus, but there are also some nice chapters on real world uses of Wordpress - pushing it beyond the blog, and using it, amongst other things, as a fully-fledged Content Management System. The book is chock-full of ideas for squeezing the most out of your installation.