"Professional WordPress - Design and Development" is really good book on WordPress. It shows you how WP works (starting from the loop, to hooking etc... which makes it outlive the WP version it was written for (I thought 3.0). I guess this is very useful for everyone who wants to take his WP skills from just running WP to hacking it to his/her own wishes.
Starting from a top-level view on WP, it then diggs down into the Loop, database, queries, taxonomies, plugin development, shortcodes, theming and, very important as WP grows: security. It also covers content building, navigation, search optimization, migration, ... more than you could wish for.
I am just a small-time developer on WP (made some 30 sites to date) but this book has been a good reference for me in a lot of darker moments. It also led me to develop a working routine that now saves me a lot of time.
Highly recommend for everyone serious about WordPress.