Pavel's Relax Into Stretch describes different methods of bypassing the stretch reflex, and gives some sample stretches.
Kurz's Stretching Scientifically gives a scientific run-down of the physiology of muscle and nerve working relevant to flexibility training, explains different types of stretching, and illustrates sample exercises. Moreover, he explains when each type of stretch should be done, and how to organise a flexibility training program, which Pavel does not.
If your aim is to gain flexibility and an understanding of stretching, Kurz's book will be of more benefit to you. Pavel's book, as it covers some methods which Kurz's does not, is a worthwhile supplement.
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