I have bought four photoshop books and taken a class so far, still a ton to learn.
So far, my votes would be:
1: Photoshop for Photographers Great book, a little advanced in parts, but very photo oriented.
2: Photoshop 7 down and dirty tricks. I got this free with my upgrade to 7.0. Clever book, a little more oriented to graphic design, but I enjoyed it anyway.
3: Adobe Photoshop Studio Techniques. This was the book we used in the class I took. Got a little complicated at times, and a lot is not photo related, but I kept it for reference.
4: Professional Photoshop 6: The Classic guide to color correction.
This is definately a pro book. It is hard to understand if you dont have prepress CMYK printing knowledge. I have started and put this book down with headaches four times. Its fascinating, and someday I will understand it, but for now its a bit advanced for my pea-brain

I think what you will find is that all of them have little bits and piecec that you will find very helpful. Most can be had at the library. Try that first, save 50 bux a book. 