While this could have served as a subset of his book about Okinawan karate, it is a very useful overview of the history of Okinawan weaponry. He makes a good case for Okinawan martial arts as essentially weapon arts that also have "empty hand" aspects, which corrects some of the more mythological and popular treatments of the subject in both English and Japanese. The influence of Chinese and Japanese systems are dealt with in detail, concluding that the influence from China is quite strong (owing to trade and diplomacy) and less so with Japan.
So in terms of weapons and general martial history this is a solid book. The Zen aspect of the title is rather misleading, as if to conflate Zen (not to be confused with Zen Buddhism) with Kobudo. If you would like to learn more about Okinawan religion, look elsewhere for that. In particular, Okinawan Religion: Belief, Ritual, and Social Structure by Lebra and Kerr's Okinawa: The History of an Island People (the definitive history of Okinawa in English). I also found his unnecessary comments about Christianity (p 57) both curiously out of place and lacking in scholarship. That the immediate followers of Jesus created a new religion obsessed with death, completely against anything Jesus was about, to "somehow appease their consciences" (whatever that means), is a very accurate description of Mark Bishop's lack of knowledge of the history of Judaism, Zoroastrianism and, in particular, Christianity, and nothing more. An odd addition to an otherwise very useful book.