For awhile now, a group of folks have been working their way through Dan Margulis' book on using LAB color mode in Photoshop and writing up a summary of each chapter as they make their way through the book. I've read the book myself, but still find the chapter summaries most interesting and useful. The latest writeup is here and is about Chapter 9 in the book. It's about using the LAB A and B channels to more powerfully or easily create masks or selections in Photoshop based on color. The big advantage to this method vs using channels in RGB mode is that LAB separates out luminosity from the color so, for some masks, it can be a lot easier. Anyway, the writeup describes it much better than I. The previous chapter summaries are here.