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.
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 buy this book at my photography store which is powered by Amazon.com great prices, fast delivery, and excellent service.