While in reclusion, Miyamoto Musashi wrote the "Go Rin No Sho", known in English as "The Book Of Five Rings", which was a text on kenjutsu, martial arts and philosophy.
Many translations of the "Go Rin No Sho" have been made over the years, and it enjoys an audience considerably broader than just those interested in martial arts. For instance, some business leaders find its discussion of conflict and how to take advantage of it to be relevant to their work.
The five "books" refer to the idea that there are different elements of battle, just as there are different physical elements in life, as is believed in Buddhism, Shintoism, and other Eastern religions.
The term "Ichi School", which is referenced in the Go Rin No Sho, refers to the "Niten No Ichi Ryu", or "Ni Ten Ichi Ryu", which when literally translated means "Two Swords, One Heaven", although the translation could be interpreted as "Two Swords, One Spirit", or "Two Swords, One Entity".
Personally, I would recommend Kenji Tokitsu's book entitled The Complete Book of Five Rings, it is hands down the best book I read on the subject. Also, check out his equally great book on Musashi entitled Miyamoto Musashi: His Life and Writings.
The Ground Book serves as an introduction, and it uses the metaphor of building a house to discuss martial arts, leadership, and training.
"Strategy is the craft of the warrior. Commanders must enact the craft, and troopers should know this Way. There is no warrior in the world today who really understands the Way of strategy.
There are various Ways. There is the Way of salvation by the law of Buddha, the Way of Confucius governing the Way of learning, the Way of healing as a doctor, as a poet teaching the Way of Waka, tea, archery, and many arts and skills. Each man practices as he feels inclined.
It is said the warrior's is the twofold Way of pen and sword, and he should have a taste for both Ways. Even if a man has no natural ability he can be a warrior by sticking assiduously to both divisions of the Way. Generally speaking, the Way of the warrior is resolute acceptance of death. Although not only warriors but priests, women, peasants and lowlier folk have been known to die readily in the cause of duty or out of shame, this is a different thing. The warrior is different in that studying the Way of strategy is based on overcoming men. By victory gained in crossing swords with individuals, or enjoining battle with large numbers, we can attain power and fame for ourselves or for our lord. This is the virtue of strategy."
-Miyamoto Musashi, The Ground Book.
The Water Book describes Musashi's style; that is, "Ni-ten ichi-ryu", or "Two Heavens, One Style". It describes some basic technique and fundamental principles.
"The spirit of the Ni Ten Ichi school of strategy is based on water, and this Water Book explains methods of victory as the long-sword form of the Ichi school. Language does not extend to explaining the Way in detail, but it can be grasped intuitively. Study this book; read a word then ponder on it. If you interpret the meaning loosely you will mistake the Way.
The principles of strategy are written down here in terms of single combat, but you must think broadly so that you attain an understanding for ten-thousand-a-side battles. Strategy is different from other things in that if you mistake the Way even a little you will become bewildered and fall into bad ways.
If you merely read this book you will not reach the Way of strategy. Absorb the things written in this book. Do not just read, memorize or imitate, but so that you realise the principle from within your own heart study hard to absorb these things into your body. "
-Miyamoto Musashi, The Water Book.
The Fire Book refers to the heat of battle, and it discusses matters such as different types of timing.
"In this the Fire Book of the NiTo Ichi school of strategy I describe fighting as fire.
In the first place, people think narrowly about the benefit of strategy. By using only their fingertips, they only know the benefit of three of the five inches of the wrist. They let a contest be decided, as with the folding fan, merely be the span of their forearms. They specialize in the small matter of dexterity, learning such trifles as hand and leg movements with the bamboo practice sword.
In my strategy, the training for killing enemies is by way of many contests, fighting for survival, discovering the meaning of life and death, learning the Way of the sword, judging the strength of attacks and understanding the Way of the "edge and ridge" of the sword.
You cannot profit from small techniques particularly when full armor is worn. My Way of strategy is the sure method to win when fighting for your life one man against five or ten. There is nothing wrong with the principle "one man can beat ten, so a thousand men can beat ten thousand". You must research this. Of course you cannot assemble a thousand or ten thousand men for everyday training. But you can become a master of strategy by training alone with a sword, so that you can understand the enemy's strategies, his strength and resources, and come to appreciate how to apply strategy to beat ten thousand enemies.
Any man who wants to master the essence of my strategy must research diligently, training morning and evening. Thus can he polish his skill, become free from self, and realize extraordinary ability. He will come to possess miraculous power. This is the practical result of strategy."
-Miyamoto Musashi, The Fire Book.
The Wind Book is something of a pun because the Japanese character can mean both "wind" and "style" (i.e., "style" meaning of martial arts). It discusses what Musashi considers to be the failings of various contemporary schools of sword fighting.
"In strategy you must know the Ways of other schools, so I have written about various other traditions of strategy in this the Wind Book.
Without knowledge of the Ways of other schools, it is difficult to understand the essence of my Ichi school. Looking at other schools we find some that specialize in techniques of strength using extra-long swords. Some schools study the Way of the short sword, known as kodachi. Some schools teach dexterity in large numbers of sword techniques, teaching attitudes of the sword as the "surface" and the Way as the "interior".
That none of these are the true Way I show clearly in the interior of this book - all the vices and virtues and rights and wrongs. My Ichi school is different. Other schools make accomplishments their means of livelihood, growing flowers and decoratively colouring articles in order to sell them. This is definitely not the Way of strategy.
Some of the world's strategists are concerned only with sword fencing, and limit their training to flourishing the long sword and carriage of the body. But is dexterity alone sufficient to win? This is not the essence of the Way.
I have recorded the unsatisfactory points of other schools one by one in this book. You must study these matters deeply to appreciate the benefit of my Ni To Ichi school."
-Miyamoto Musashi, The Wind Book.
The Void Book is a short epilogue, describing, in more esoteric terms than the other books, Musashi's probably Zen-influenced thoughts on consciousness and the correct mindset.
"The Ni To Ichi Way of strategy is recorded in this the Book of the Void.
What is called the spirit of the void is where there is nothing. It is not included in man's knowledge. Of course the void is nothingness. By knowing things that exist, you can know that which does not exist. That is the void.
People in this world look at things mistakenly, and think that what they do not understand must be the void. This is not the true void. It is bewilderment.
In the Way of strategy, also, those who study as warriors think that whatever they cannot understand in their craft is the void. This is not the true void.
To attain the Way of strategy as a warrior you must study fully other martial arts and not deviate even a little from the Way of the warrior. With your spirit settled, accumulate practice day by day, and hour by hour. Polish the twofold spirit heart and mind, and sharpen the twofold gaze perception and sight. When your spirit is not in the least clouded, when the clouds of bewilderment clear away, there is the true void.
Until you realise the true Way, whether in Buddhism or in common sense, you may think that things are correct and in order. However, if we look at things objectively, from the viewpoint of laws of the world, we see various doctrines departing from the true Way. Know well this spirit, and with forthrightness as the foundation and the true spirit as the Way. Enact strategy broadly, correctly and openly.
Then you will come to think of things in a wide sense and, taking the void as the Way, you will see the Way as void.
In the void is virtue, and no evil. Wisdom has existence, principle has existence, the Way has existence, spirit is nothingness. "
-Miyamoto Musashi, The Void Book.