Check out everything you ever wanted to know about Stretching and Flexibility by Brad Appleton. Basically, this is a free book which takes you through the mechanism of stretching from a more scientific point of view. It covers pretty much everything from an intro to the musculoskeletal system to discussions on what muscles are, various types of muscle fibres and what happens when you stretch. Then, there is info on different types of stretching – balistic, dynamic, passive etc etc and also a good section on how to stretch. Warm up and cool down routines are covered, as well as normal range of motion information for various body parts. No actual diagrams, but well written, this could keep you busy for some time and makes a useful reference for anyone undertaking any sort of flexibility training. Great stuff!