The Book of Five Rings

Author of this book – a famous Japanese samurai Miyamoto Musashi (c.1584–1645) who considered to having been one of the most skilled swordsmen in history. He participated in duels from a very young age and became legendary through his outstanding swordsmanship. He is the founder of the Hyōhō Niten Ichi-ryū or Niten-ryū style of swordsmanship. In this book, he shared his thoughts on strategy, tactics, and philosophy.

Go Rin No Sho is the teachings that Musashi presented to his students in his own dojo. Despite a certain borrowing of ideas, the text is largely the original work of the great master. Depends on what you are looking to learn on strategy. 

While Art of War by Sun Tzu is technically based on military strategies on armies fighting each other, 'The Book of Five Rings' is about confrontation between people. They complement each other perfectly.