Professional WordPress teaches readers about the inner workings of WordPress starting with three chapters near the beginning of the book before covering theming and plug-ins in the middle chapters and then going into practical examples that round out the book. The main chapters focus on: navigating the code-base, the loop structure, and data storage.
Things get started with the forth chapter that gives a overview of the code base. While WordPress' own Codex wiki is mentioned, it's pointed out that it is not always up to date. Personally I found the Codex was a bit dis-organized and last time I looked through it, and had a to google a lot of items to find answers. That said, this book does have a lot of stuff that you could did out of the Codex, but it's generally better presented here. One of the good parts in this chapter is a overview of the files found in a WordPress installation and the major functions in each; like any large code-base that has been in constant development for nearly a decade, a guide like this is invaluable for figuring out where to start understanding the code- I wish other books on large frameworks took a similar approach.
The next chapter covers the Loop that drives WordPress' public-facing display. While at the most basic it simply lists blog entries, digging just a bit deeper into the API shows that it can be manipulated to do a lot more than lists blog entries chronologically one after each. There have been a few books written about controlling the loop, including recently Thord Daniel Hedengren's Smashing WordPress, but this book really goes into all the options theme designers have available to them in a systematic way.
Finally, most books don't really touch the database structure that powers WordPress. Chapter six covers everything from a break-down of the database structure, how taxonomies are stored, and the various API commands WordPress has for communicating with the database as well as some tips on working manually with the database using PHPMyAdmin. With the internals covered, the book then goes into creating themes and plug-ins.
Though the book does start with the obligatory "how to install WordPress", it's really much more about how WordPress is put together from the database-level on up. Even the installation chapter goes into a lot of depth about understanding the config.php file and how what the various options are. As readers would expect, there's plenty of PHP code to be found, but there isn't an over-arching project. The book is written for version 2.8 so the API information is up to date (at the time of writing this review, v2.9.2 is the vary latest release). At 300-odd pages it's not a huge book, but for developers looking to get into programming WordPress there's a lot of well-organized material packed into it.