Book of 5 Rings - Ground Book
The ground book () describes the whole book itself and the key points of Musashi's school of martial arts.
Musashi explains that carpenters makes use of every kind of timber, even those that appear useless. The measure carefully and maintain their tools well. Similarly, he recommends intelligent use of resources.
In explaining the the Two Sword technique he describes its usefulness when facing multiple enemies. This is an interesting and recurring theme in the book. Musashi assumes you are going up against a numerically superior force. This makes his ideas particularly useful if you are outnumbered.
Next Musashi talks about rhythm. This means understanding cycles of growth and decline and timing generally.
Lastly he explains nine rules for studying his path. This is one of the great parts of the book because each one could easily be made into a book itself. If only modern business manuals condensed their knowledge this effectively. Musashi used a very short form of expression here so each character had many implications. Accordingly you need to think deeply about each point here. None of them have only a superficial meaning:
Practise the martial path assiduously
Get to know the arts
Learn the ways of craftsmen
Discern the cost and benefit of all things
Develop your perception
Perceive the things not visible to the eye
Pay attention, even to small matters
Don't do useless things