This book is rightfully a classic. I am not sure if the insights here that Sun Tzu offers us are common sense, or perhaps they just sound like it after you read it, but the book definitely has some very straight forward ideas about the most direct and useful military tactics.
Sun Tzu outlines what you need to understand to attain your goal. You need to know things like your terrain, your equipment, your troops, and your enemy.
If I may be permitted to be philosophical here for a moment, I’d like to speculate on how this book became the classic it did. The secret, I suspect, lies in the fact that the advice in the book has little to do with war. Sure, the book is about war, but in reality, the book is far more generalized. In some sense it reminds me of Hilbert’s axiomitization of geometry. Hilbert has a famous line about his geometry being not only about points, straight lines, and planes, but also about anything that meets the criteria of the axioms – even tables, chairs, and beer steins. The Art of War is the same way. Sun Tzu could be talking about business competition or how to pick up women and the advice would be the same. Instead of "enemy troops", you would have "competitor’s product" or "woman in bar". The ability to give advice, no matter how obvious, that is this general and universally applicable, is surely the mark of a careful and penetrating thinker.