Secrets of the Pelvis for Martial Arts: A Practical Guide for Improving Your Wujifa, Taiji, Xingyi, Bagua and Everyday Life, by Michael Buhr.
This book looks really interesting, pretty good reviews on Amazon. Anybody read it and/or know the author?
Buhr also has a blog at http://internalgongfu.blogspot.com/
I REALLY liked his most recent post. Gonna have to delve further.