Must be a classic already? I can't add much too all the positive reviews listed here already, but, despite being a small book and a quick and easy read, it's one of those books that should be read and its contents digested several times, to make sure the important messages have sunk into your sub-consciousness.
Joe gives several accounts of when he trained with Bruce Lee and there are many witty and funny stories in the book. The book covers many Budo secrets and is a fantastic summary of many of the great samurai classics. No matter what Martial Art you study, there will be plenty of material here to improve your performance and help you with the important aspects of breathing, timing and Kime.
Try and get the Bantam July 1982 edition as it is nice and small and can fit in your pocket. Keep it by your bedside to give you something to meditate on before you go to sleep and refer to it often to help you absorb the important techniques within the book. If followed with some persistence, I am certain it could help anyone increase their longevity though the various practices in the book.