Favorite Yoga Books?

I just bought Light on Yoga by BKS Iyengar that my yogi told me is the classic book on Yoga. So far so good - all the poses are rated from one to sixty in difficulty with one being easiest and sixty being most difficult. But even better is the introduction. Just started reading it last night and did a quick look through first and now I'm settling into the Introduction. What is your favorite yoga books?