I really like how David Kiel Anatomy talks. Here he discusses Janu Sirasasa and my nemesis pose, Setu bandhasana. I would highly recommend his adjustment DVDS too (if you teach and like to adjust students of all abilities).
This yoga manual is a fantastic reference book for both yoga teachers and students.
The author - H. David Coulter - has a ph.D in anatomy and has been a long-standing yoga practitioner, and has trained and studied with a number of yoga Swamis.
I think this book is really insightful and clearly links the relationship between the practice of yoga and the human anatomy.
It is full of information; including the mechanics of breathing techniques, Asana, Mudra and Bandha from both a Western and yogic perspective. I have not found another book that crosses into both areas in quite the same way as this one does.
My husband who does not practice yoga, but does have an MSc in Sports Medicine also thought it was a great comprehensive reference book.
Anatomy of Hatha Yoga: A Manual for Students, Teachers and Practitioners by H. David Coulter