Miyamoto Musashi (c. 1584-1645) is the most revered and celebrated swordsman in Japanese history; unfortunately, our modern portrait of this folk hero is derived mainly from popular books, comics, and film, with little heed paid to the early records by men who knew Musashi, practiced with Musashi, and went into battle with Musashi.

Spanning a period of more than a decade, the author set out to translate all surviving records on Musashi. The result is a body of text comprising some 150,000 words, mostly written during Japan's feudal era. They range from original accounts of...