As a graphic designer who relies heavily on Photoshop, I've always found the Photoshop WOW! books to be great resources.

Here are a couple other books you may want to look into, as well:
Photoshop Restoration & Retouching, Second Edition
Photoshop Channel Chops
Photoshop Studio with Bert Monroy - Props to Tony for mentioning Bert. His work is astounding, although he uses Photoshop and Illustrator in conjuction, in many cases. Still much to be learned.
Photoshop 7 Artistry: Mastering the Digital Image
Photoshop 7 Killer Tips

I have more if you like, but those should keep you satisfied for a while.