Simply put, “device independent” color space is one that regardless of which device is used the human eye will always see the exact same color.
RGB an CMYK color spaces are component based spaces. Whet it means is that when a color is “created” it is made up of certain amounts of each component. The problem is that different devices use different components. Take for example two monitors. Their RGB phosphors will be different, so despite the fact that they may display same quantities of RGB components, the human eye will see two different colors. The same is true for CMYK – take two printers, and same CMYK amounts will result in the eye seeing two different colors.
That is not the case with LAB. When the same LAB value is measured on any number of devices (which will probably require different component, RGB or CMYK, to do that, which is what device profile do), the human eye will always see exactly the same color.
A great book to read about Photoshop LAB color is Dan Margulis’s “Photoshop LAB Color: The Canyon Conundrum and Other Adventures in the Most Powerful Colorspace”
Which makes it the most powerful color space!
This book is outstanding! It’s not for beginners, and it is not an easy read for advanced user either – you need to have a solid understanding of what you’re doing with RGB or CMYK color adjustments, and basic retouching first. Working in Lab is an incredibly powerful way to pull out very natural and vibrant colors with much less effort.
Dan Margulis’s knowledge of color runs deep. This book is nothing short of amazing. If color matters to you as it is to me, this is a must read book.
Dan essentially advocates color correction and image improvement through LAB, and he advances compelling evidence to support the concept. The trick is to get a grasp of what kinds of images LAB excels at, and where it falls short (in which case Dan points out which of the remaining color spaces, RGB or CMYK, is the better option). He also demonstrates image correction in as few steps as possible, often without selections (rather, he generates what he needs by employing more useful channels… which helps avoid obvious selection edges that do not match).
As I said, it’s not an easy read. I keep picking it up and read out a few pages here or there, trying out techniques as I go along. And by now I use LAB almost exclusively for color corrections.