While I agree that you can learn much of the information in Curly Girl: The Handbook online, I still think the book is quite worthwhile. For one thing, it does define Curly Girling, because, yes, the author did come up with the method.
If you just skip around, and read the parts you think apply to your hair, you can miss a lot. Even after CGing for years, and having read the original book when I started, I found the current (second) edition worthwhile, and learned a lot from it. I don't find the curl classification system at all helpful, so I was willing to try suggestions for other hair types, and found a number which were helpful, thanks to reading from cover to cover.
I also think that the illustrations and videos are very clear and easy to follow. While you may be able to pick up on some of it on YouTube, the photography, clarity, and succinctness of the videos on the accompanying DVD makes them really stand out, and make it much easier to get a full understanding of the techniques presented. The section on trimming your own hair is IMO worth far more than the price of admission. (I will never again lose years of growth because some idiot doesn't have enough sense to realize s/he hasn't a clue how to cut curly hair, or be placed in the humiliating position of leaving a hair salon looking like I've been hit with a weedwacker.)
Tiffany Anderson's book, Live Curly, Live Free, is also extremely helpful. The information is definitely more technical and scientific. Personally, I think the two books compliment each other, and neither is expensive. For a combined total of something like $20, I got both, and I'm happy to have both of them, but I'm glad that I started with Curly Girl.
Baby Fine 3B, low porosity, normal density and elasticity
CGing since July 2008