Sun Tzu

Sun Tzu's thirteen short chapters cover everything from terrain and supply lines to spies and the psychology of a frightened enemy, built around one central idea: the best victory is the one where the fighting was never really necessary. Every maxim is built for reuse outside the battlefield it was written for - assess before you commit, know your opponent better than you know yourself, never fight the battle your enemy wants you to fight - which is exactly why it ended up on the reading list of generals, chess players, and corner-office executives alike.
The oldest and most influential military treatise ever written, and one of the rare ancient texts that reads less like history and more like a manual someone is still actively using.