There are lots of good Photoshop books, and that’s probably another. However, I find many of these books don’t focus on the kinds of things you want to do on the web. So you might like to consider this:
Also, there are millions of great tutorials online for specific tasks. For example, Google something like “creat web backgrounds in Photoshop” or “web 2 buttons in Photoshop” and you’ll get lots of good guidance.
I would agree with Zippers, the “WOW” books are pretty good for getting going fast, I had one for illustrator and it was so good I gave it to a friend once I was done with it because it will help them get going fast
I like those because because they get you practicing off the bat and you make cool things and remember them quickly.
try those free tutorials online to learn photoshop. there are a ton of demonstration videos online. that is how i learned to use it other than trial and error… and believe me, you will have those trial and error days!
Another good book to look at is the Classroom In A Book by Adobe http://www.amazon.com/Adobe-Photoshop-CS4-Classroom-Book/dp/032157379X . It’s not specifically about web design, although it does cover web graphics but it is very thorough and what you learn can be applied in any kind of design work. I use the classroom in a book series as a resource for all of the classes. They’re really good.