Any history calling itself "authentic" posits one that is inauthentic. Here the target is apparent. It is the "Miyamoto Musashi" of Eiji Yoshikawa, published 1935-39 and translated into English as "Musashi" in 1981.
Still in print and going strong, Yoshikawa's best-seller achieved its popularity by cleaning up its hero, giving him an idealistic education and assuring that he was the poster-boy bushido-type warrior Japan needed in 1935 at the beginning of its final military adventure.
This new and "authentic" Musashi, however, is quite dissimilar. He is reported to be a "stinking wretch who actually boasts that he never takes a bath," who practices an "inelegant, meat-chopping style" of swordsmanship, and would seem to believe that "nothing can beat the look on a man's face when he sees his cock and balls go flying into space."