My best recommendations are for Sport Stretch and Stretching Scientifically, followed by Health & Fitness Excellence, SynerStretch, or Stretch and Strengthen. Mobility Training for the Martial Arts also has quite a bit of valuable information and stretches. The Health for Life Training Advisor has a lot of information about stretching and muscle physiology, but it is not strictly about stretching and contains a ton of other information about all aspects of athletic training and performance (which I find to be invaluable). If you don't want to get into too much technical detail and are looking for a quick but informative read, then I recommend Stretching Without Pain. If you really want to delve into all the technical aspects of stretching, including physiology, neurophysiology, and functional anatomy, then you must get Science of Stretching. If you want to know more about PNF stretching, then Facilitated Stretching is the book to get. If you are looking for yoga or active stretches you simply must take a look at ExTension (also your local library probably has quite a few books and/or videotapes of yoga exercises). If you want to know more about muscle anatomy and physiology but don't have a lot of technical interest or background in those two fields, The Muscle Book is highly recommended.
Many of the other books don't have as much detail about stretching and what happens to your muscles during a particular stretch, they just present (and illustrate) a variety of different exercises. Also, most of the stretches presented in these books are to be performed alone. Sport Stretch, SynerStretch (both the videotape and the book), and Mobility Training for the Martial Arts present stretches that you can perform with the assistance of a partner.
In general, Health For Life (also known as HFL) and Human Kinetics Publishers have a tremendously wide variety of technical, no-nonsense, exercise related books and videotapes. I would highly recommend contacting both organizations and asking for their free catalogs:
Human Kinetics Publishers, 1607 North Market Street, P.O. Box 5076, Champaign, IL USA 61825-5076, 1-800-747-4457 (US), 1-800-465-7301 (Canada)
Health For Life, 8033 Sunset Blvd., Suite 483, Los Angeles, CA USA 90046, 1-800-874-5339