There's a ton of great books, but if you could identify what areas you're most interested in, we could suggest books which are tailored towards your goal.
F'rinstance, while Deke McClelland's
Photoshop Bible is an excellent book on the topic, it's not--in my opinion--a good choice for photographers. It doesn't address color correction until 800+ pages into the book, and that's one of the first things a photographer needs to know.
Photography oriented books are similarly bad choices for graphic artists.
Where do you want to go
first with the tool? If it
is photography, take a look at my
list of recommended Photoshop books. If it's something else, ignore that list entirely.