Beginning WordPress 3 is outdated! You should get the new edition instead: WordPress for Web Developers. (It’s the same book, only updated; we changed the title to make its intended audience clearer.)
Beginning WordPress 3 is intended for developers who are just getting started with WordPress. Once you’ve finished it, you’ll need some more advanced resources.
Professional WordPress, by Hal Stern, David Damstra, and Brad Williams, includes an overview of the core code and database schema that would be great reading for anyone who wants to begin contributing patches for inclusion in WordPress. It also discusses how to use WordPress for large-scale sites, including scaling, performance, and maintenance.
WordPress Bible, by Aaron Brazell, is a huge reference of all things WordPress. I found its organization a bit confusing, but there’s no better source for advanced topics like server tuning and caching. It also includes some great jQuery examples and a thorough discussion of the database and query classes. While the book was published before 3.0 was released, it does cover WordPress MU 2.9.x. Very little has changed in 3.0, except that MU is no longer a standalone product.
Digging Into WordPress, by Chris Coyier and Jeff Starr, is packed with short, easy-to-digest tips for plugin and theme development.
On the web
- I used
query_posts()to simplify the multiple loop examples, but you really ought to use the WP_Query object instead.