It's clear from the questions I receive that many people want to improve their knowledge of flexibility training, but they are unsure where to start. The following is a list of books that I consider to be 'must-haves' and I recommend they are read in the order they appear. However, remember that this list is intended to provide a starting point; you are encouraged to delve further into the literature to learn more about specific points that interest and intrigue you.
1. The Complete Guide to Stretching, 4th Edition (Christopher M. Norris)
Written by a physiotherapist and lecturer, this book provides a simple introduction to the subject of stretching, while also touching on important but often complex topics like biomechanics. It lists plenty of references for you to sink your teeth into should you wish to explore the cited literature in more detail.
2. Stretching Scientifically, 4th Edition (Thomas Kurz)
If you are well versed in reading scientific articles, start here. Otherwise, you may end up feeling overwhelmed - Kurz is an expert in his field but his writing can be cumbersome for people new to the subject. While the book does provide plenty of references to the literature, it is a follow-along text at heart - you can skim over the science-heavy parts and jump right to the routines if you just want to know the exercises you should do and in what order.
3. The Science and Physiology of Flexibility and Stretching (David Behm)
David Behm is one of the foremost researchers on the subject of flexibility training. It was he, along with his co-authors, who presented evidence that showed doing static passive stretches during a complete warm-up is not harmful to performance. This contrasts strongly with Kurz's book, which very much drives home the message that static passive stretching in a warm-up is useless and potentially harmful. I too used to teach that static passive stretching should be saved for the end of a workout (or for separate training sessions entirely) because Kurz was a considerable influence on my career as a coach and educator (he was my mentor for many years). However, science changes knowledge over time, and static passive stretching in a warm-up is certainly not as disastrous as Kurz suggests.
4. Basic Biomechanics, 9th Edition (Susan Hall)
Biomechanics is an area that is integral to the study of flexibility, but it can also prove extremely difficult to understand due to the technical proficiency required to grasp even its rudimentary elements. Hall's textbook (now in its ninth edition) provides an easy-to-understand entry into the subject, and it does a better job of it than Biomechanics for Dummies.
5. Neurophysiology: A Conceptual Approach, 5th Edition (Roger Carpenter, Benjamin Reddi)
Like biomechanics, neurophysiology is an area that is fundamental to knowing how and why the body responds to flexibility training inputs like stretching. In some ways, it can be more complex than biomechanics owing to the intricate nature of many of the neural structures that contribute to important processes like the stretch reflex. This book does an excellent job of breaking down barriers to learning and it provides a useful entry point to this fascinating subject.