This book was written to answer the perenniel question, "What's the difference between layers, selections, channels, and paths?" It was written for CS2, so some of the content is now out of date.
Here is a link to the files
referenced in the book.
Graphic designer, teacher, and Photoshop expert Nigel French saves you hours of frustration by illuminating the best selection tools and techniques for specific tasks, and the most flexible approaches to correcting and editing images in Photoshop. Using step-by-step examples, this book clarifies often-misunderstood selection methods, shows which masks best select a range of tone or color, and demonstrates how the right tools and techniques can help transform even the most mediocre picture into a polished image, saving you hours of time.