I've enjoyed all the fictionalized accounts, but I think that if you want to learn from Musashi, you should read him directly. That's the Go Rin no Sho aka The Book of Five Rings. Nekopa has recommended the Cleary translation. I will recommend Hidy Ochiai's version, A Way to Victory: The Annotated Book of Five Rings. The annotations and analysis (after each book) really help get a sense of Musashi. Some points that stood out to me, considering that I'd read several other translations before:

His absolute priority on winning. It's not that his style is the best way to win, it's that his style is based around winning. The whole point of the book is winning! You always have to win, by any means necessary. It's very bald; the text is peppered with bits like

A tenet of my teaching is that one must win regardless of the length of the sword one uses, so I do not dictate the length of the sword. The ultimate objective of my style is to be prepared to win, no matter what weapons are involved.

and somewhat disconcertingly,

When applying martial strategy to the world of leadership, it is important that you make the acquaintance of people of good character; that you become a good leader to others; that you conduct yourself in a correct manner; that you govern people well; that you take good care of others; that you follow and maintain the laws and customs of the land for the sake of order; and that you never take second place to anyone in whatever you engage.

It's also noted that while Musashi did take on well-known opponents, especially Sasaki Kojiro, he also never dueled with several other extremely famous samurai in the same period. Ochiai notes that it is entirely consistent with his strategy to not fight if you think you will not win.

Directness and simplicity: As I sit down here to begin writing this book, I do not intend to use any archaic words from the scriptures of Buddhism or Confucianism, nor will I depend on the examples of various writings of old war chronicles and battle tactics. Musashi also considers the idea of dividing techniques into basic and advanced stupid, although he concedes that you should teach what's easiest for a beginner first, depending on the person.

Humility: However, after the age of thirty, I started to reflect on my experiences and began to wonder whether or not my victories were attributable to my natural ability, sheer luck, or the inferior techniques of those whom I had defeated, rather than my true understanding of the principle of martial strategy.

Once I became enlightened by the true meaning of martial strategy, I ceased to have any real interest or desire in the worldly affairs.

Strangely individualistic: Ochiai writes, "It’s true that Musashi intended to teach his readers to be victorious in one-to-one combat as well as in war involving armies. But if you read the book carefully, with an open mind and sincere attitude, it becomes clear that Musashi’s teaching goes far beyond that. His philosophy can be summarized as ji-riki; the power of each individual that emanates from within oneself. Musashi believed that ji-riki must be cultivated and empowered through constant effort and training. With correct understanding, his messages become concrete and personal, directly relating to the problems of living – to the human being who struggles and strives for individual achievement."

Spirit of discovery: It is not enough that you read what is written here, you must train as hard as if you were the one who developed the doctrine, instead of being the one who had it given to you. Train constantly as if you were the source of the discovery of the Way. Avoid mere imitation or learning without sincerity.