Wordpress - Recommendations for an In-Depth WordPress Book?

Another good one is Digging Into WordPress by Chris Coyier & Jeff Starr

http://digwp.com/book/

EDIT 9/2/10

I wanted to add another option which is WordPress Bible by: Aaron Brazell with Mark Jaquith Technical Editor:

I own this book and it is the closest thing to a complete WordPress reference thats available.

Until now, there has never been a book that covers the topic holistically and exhaustively and for all levels of expertise. -Aaron Brazell

If you are like me you probably view most books as being much too simple to really learn anything useful from because publishers are always trying to dumb them down to reach a broader audience. If you are like me, these two are probably some of the better books on the market because I think they are among the more advanced:

Build Your Own Wicked WordPress Themes

_{(source: mikeschinkel.com)}

WordPress Plugin Development – Beginner’s Guide

_{(source: mikeschinkel.com)}