Eight chapters by four experts on theming WordPress.
Wordpress
Professional WordPress
Design and Development
Out of the over 300 books on WordPress, few get down into the into WordPress’ code itself. Professional WordPress is that kind of book.
Smashing WordPress
Beyond the Blog
Make your next WordPress site a smashing success (And make sure it doesn’t get smashed.)
WordPress 2.7 Cookbook
Get things done with WordPress without writing a lot of code.