A couple of months ago I picked up a book by Martin Evening titled, Adobe Photoshop CS2 for Photographers -- "A professional image editor's guide to the creative use of Photoshop." I heard about the book through a Daily Shooter interview with my favorite photoblogger, David Nightingale (take one look at his pics and you'll immediately see why I say that if this book is good enough for him, it's good enough for me -- no one inspires me more).
If you are serious about digital photography I think you kind of have to buy this book. I've been using Photoshop in various capacities for 10+ years, but never really needed to draw on all of its power until I started getting into photography. This book really leaves no stone unturned and offers a lot of practical, real-life solutions to problems we all face when post-processing our images. I highly recommend it.