Parts of the book make references to Chinese classics and sayings that would be obvious to educated Japanese of that era. It would be similar to advise, "above all, be true to yourself." Anyone would know you were paraphrasing Hamlet.
Similarly the book's name and structure is a reference to the Buddhist elements. The Buddhist cosmology recognises five elements: Ground, water, fire, wind, and emptiness. These are symbolised in the Japanese, Tibetan-style Stupas. The Tibetan characters on the side are the names of the elements. The one on the right is recently made and standing in front of a Buddhist temple. The one on the left is from Musashi's period. These stupas are called Five Ring Towers (Go Rin To - ) while the book is literally five Ring Book (Go Rin Sho- ).
Here's the whole thing in the original Japanese
A deeper analysis of the book itself:
Book of Five Rings - One ring at a time