Photoshop is like learning to play guitar-you'll never know everything there is to know about it. To start, I'd suggest a few youtube tutorials, and maybe even a class at your local city college. I finished college in 2006, so I'm not really sure of any books to recommend.
Once you get a feel for the basics, it's pretty fun to develop your own style and see what you can do. Good Luck and keep asking questions!
<-----Graphic Artist
(me, not chris farley)