Anatomy of a Street Fight
By Paul Vunak
A great reference book for PFS and JKD training.
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Filipino Martial Arts
By Dan Inosanto
Unfortunately, this is out of print, but it's a collectors dream. The cheapest I have seen is about $100 U.S.
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Wrestling for Fighting: The Sport of Mixed Martial Arts
By Randy Couture
I would suggest you have a understanding of wrestling or ground work to get the best from this book but should be part of any serious martial artist library.
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The Bible
The most sold book in the world and should be read by all. I particularly like the Gospel of John and Romans