I've recently been reading "The science of yoga" by WIlliam Broad. I'd really recommend it if you want to know more about the science behind yoga. It demolishes a few myths and talks about the risks of yoga as well as the benefits. A really balanced book written by a yoga practitioner. It's well written too, I'm already on my second read as it's full of little gems that I can use in my teaching and my practice.
Also on my recommended reading list is "The wisdom of yoga" by Stephen Cope. He sheds light on the ancient yoga aphorisms (or sutras) of the sage Patanjali by using stories from real lives to illustrate them. I confess I've struggled with the philosophy - it doesn't come easily to me. But this book is really opening my mind.