Was Musashi Miyamoto a real personality and is Yoshikawa Eiji's "The Book of Five Rings" his real biography?

Question:

I would like to learn more about Miyamoto Musashi. Where can I read the real biography?

Answer:

Yes, Musahi Miyamoto was a real personality.

"The Book of Five Rings" is not a biography.
It's written NOT by Yoshikawa Eiji, but it was written by Musashi Miyamoto. Musashi wrote "The book of five rings" to hand down his swordsmanship and his secret doctrine to posterity. People believe that his dictrin is still good for mordan society although we have no need for swords nor battle.

Yoshikawa Eiji's books are novels. It based upon true stories, but he added some fictional parts. Romantic feeling for the lady named Otsu is one of that.
However, I guess it's good starting points to read his novels to understand Musashi. Most of Japanese people's understanding what Musashi was like, is based upon established images by Yoshikawa's novel.