While layers are the most often used feature in Photoshop they have been completely
ignored as the subject for a book, until now. Richard Lynch, author of 7 books on Adobe
Photoshop and Photoshop Elements, has compiled a complete treatise on using layers
to improve your photos, streamline your workflow, retain your corrections for later editing,
adopt a non-destructive workflow, and explore your creativity.
This could be the single best book for enhancing your investment in Photoshop.

The Adobe Photoshop Layers Book has chapters on:

Chapter 1: The Basics of Layers: Layer Functions and Creation

Chapter 2: Layer Management: Concepts of a Layer-Based Workflow

Chapter 3: Basic Object and Image Area Isolation

Chapter 4: Masking: Enhanced Area Isolation

Chapter 5: Applying Layer Effects

Chapter 6: Exploring Layer Modes

Chapter 7: Advanced Blending Modes

Chapter 8: Breaking Out Components

Chapter 9: Taking an Image Through the Process

Chapter 10: Making Layered Collage and Composite Images