A somewhat short treatise on the way of the warrior is always welcome, particularly when coming from a guy who's essentially Alexander in one-on-one combat. Musashi slew his first man at 16, and did not slow down for the rest of his life. He's also lead some campaigns and has insights on strategy, but these don't seem to be as interesting or insightful as the one-on-one swordsmanship he writes about.
The book is split into Earth, Water, Fire, Wind and Void. A way to explain each would be the following: Earth is the swordsmanship and war book. Water focuses on spirituality, calm and introspection. Fire is "how to kill dudes deader" with very specific instructions on swordplay. Wind is a book on intelligence gathering and perceptiveness, and the most interesting is the latter, book of the Void.
The book of the Void is a warrior poet's approach to epistemology and self-knowledge, and I'm very impressed by it. In essence, it has to do with the pipeline of unknown unknowns all the way to unknown knowns. The most important this that this book teaches is the difference between soldier and warrior: the former is what you do and the latter is what you are.
In closing, I think this is one of the best works I've read in a long time - I shall close with the quote that stuck with me, as it completely encapsulates my brain's functioning in a simple sentence.
"Learn the Way, and then you will see the Way in all things."
9/10