"The Photoshop CS2 Book for Digital Photographers" by Scott Kelby is the best that I have seen as of yet.
Everything is step by step. The instructions involve some advanced photoshop techniques, but I think that he explains them well enough for anyone to be able to do.
other than the site mentioned.... you can just google a specific technique you wish to learn like 'layer masks' etc..... 9 times out of 10 you'll find a tutorial..... then just bookmark em or archive em.... its the cheapest option!