五輪の書 is named for its five scrolls, and the rings are the scrolls. 五輪 is a Buddhist word for the five elements that make up the world. They are the same five that stack up in the five-ringed stone towers of old graveyards. Musashi gave one scroll to each. The dictionaries write the title 五輪書, without the particle. The scroll writes 五輪の書 with it, as the 1915 edition printed it, so both forms are right.
五輪の書 is composed of the kanji 五 (read go) meaning "five", 輪 (read rin) meaning "ring", the grammatical element の (read no) meaning "of" and 書 (read sho) meaning "book; writings".
The five scrolls are Miyamoto Musashi's, written from 1643 to 1645 in his last two years, but the book never calls itself 五輪書, only a book in five scrolls, earth, water, fire, wind and void; the title is a later name for it, already in use in a Kumamoto biography finished in 1776 and in print from 1886.
We want you to be as happy with the meaning as you are with Master Takase's art. To learn more, visit Philology of 五輪の書.