Sun Tzu, The Art of War. But beware of bloated commentary translations that make it into something that it is not. Get a very tiny version, such as the Thomas Cleary Pocket Edition. The brilliance of this book is that it distills strategy down to a timeless and easy to comprehend essence, and it was written around 2500 years ago by some unknown Chinese general. Sure, it frames strategy in military terms, which isn’t always the best, but it puts it in context. War is a necessary skill leaders must take up out of necessity and for survival, so you might as well kick ass when you have to. Only pick fights you can win, and win the fight before even walking into it. Thanks to good information and planning, victory should feel like throwing rocks on top of eggs, and all that sort of good stuff. There’s plenty of other good strategy books, but few others are so tiny, well organized and timeless.