Depends on what you want to do. I've owned a few books over the years:
Interface Design With Photoshop.
The Photoshop WOW! Book (there's a whole series of these. The Photoshop one is pretty decent)
Photoshop Type Magic (I think that was the title). Lot of cool effects you can do to your type.
If you can, you might see if your local community college offers any free training classes in Photoshop. I learned a lot from playing with Photoshop on my own and from working with it in my current job, but I've also taken a few free classes that helped a bit. Right now, I'm in a free 18-week night class paid for by the state of California (long story) that covers Photoshop, Illustrator, Flash, and Dreamweaver. I do a lot of web design as part of my job already, but wanted to learn Illustrator and Flash in addition to bulking up my Photoshop skills a little.
EDIT: I don't know if you can get a hold of this if you're not a student or otherwise affiliated with an educational institution, but Adobe has a book called the "Photoshop Classroom In A Book", that we used during the Photoshop portion of the class I'm in currently. Pretty good book as well that covers the major areas of Photoshop.