Conversation so far:
What were some of Miyamoto Musashi's teachings?
Miyamoto Musashi created and refined a two-sword kenjutsu technique called niten'ichi or nitōichi or 'Niten Ichi-ryū'.
Did Miyamoto Musashi write any books?
Musashi's most famous book is the The Book of the Five Rings.

Question: What was the book about?
Self-contained rewrite: What was The Book of the Five Rings by Miyamoto Musashi about?