KrisM86:Ina May's Guide to Childbirth by Ina May Gaskin. It's not a method book per say, but she includes a lot of great information about birthing naturally as well as many birth stories. I found this book to be the most helpful out of everything I've read on natural childbirth.
This one is great, it actually got me excited to have another baby! I also like "Hypnobirthing the Mongan Method," but I was doing Hypnobirthing, so that's probably why.