Everything you need to know about Dan Margulis is in Mastering Photoshop: Advanced Color Correction, Part One. After you have devoured the book I recommend in that article, you will know why you need to get Photoshop LAB Color: The Canyon Conundrum and Other Adventures in the Most Powerful Colorspace (2nd edition).
David Biedny, author of Photoshop Channel Chops (another book well worth owning), called Photoshop LAB Color “probably the most important Photoshop book ever written.”
Scott Kelby, President of the national Association of Photoshop Professionals, wrote “This book will radically change how we do color correction . . . it’s that revolutionary.”
You can also find the first edition at a great price in the used market place.
The Mastering Photoshop Series
The “Mastering Photoshop” Series – Series Overview
Buyer’s Guide Series
The Mastering Photoshop Series is part of a larger series of over 60 articles that guide you to the best of all things photographic. The rest of the series is here: Buyer’s Guide: Recommendations For The Best Photography Equipment, Software, Books, Magazines, DVDs, Online Photo Labs and More.