On the primary texts. Read Go Rin No Sho and Fudōchi Shinmyōroku themselves, in any competent modern translation, and read them after this book rather than before. They are short. Between them they are an afternoon.
Compare two translations of the same passage. It is the fastest way to see how much of what you are reading is a translator's decision. Where one says rhythm, another says cadence and a third says timing, and all three are 拍子.
What to be careful of. Free scans of the Japanese are widely available, and a large share of the ones that circulate are 1941–43 wartime editions. Those are not neutral texts; they were printed by a state that had a use for them, and Part Four's chapter on the twentieth century is about exactly that use.
And any edition that promises the book will make you a better negotiator has told you what it has done to the material before you have opened it.
The thing this book will not tell you. Whether any of it works. Both men are explicit that what they are describing takes decades, cannot be transmitted by reading, and looks — when it is finished — exactly like a person who never began.
They wrote it down anyway.