If you are a tutorial learning type person “Photoshop CS2 Classroom in a Book” is a good place to start. It comes with a disc that includes the same images the book uses as examples and walks you step-by-step through each tutorial. The first couple of chapters outlines what each tool does and how it is used and then moves on from there. It’s a good book to “make you dangerous”.
If you come from a traditional photography background, another great book is “Max Ferguson’s Digital Darkroom Masterclass”. It is a much more advanced book, but explains the tools in Photoshop by equating them to traditional darkroom techniques. This book is not going to “teach” you Photoshop, but is great in terms of the theory and principles as to why certain tools work the way they do.
Both books run anywhere from $30-$50, depending on where you buy them.