One of the best books I've read and continue to use as a reference is The Art of War. This ancient text written by Sun Tzu on war is now used by business men throughout the world. If you are in school, at work, or at home, you can find something in this book that can help you with whatever problem you might have. In this book you also have commentaries from other great figures that followed Sun Tzu: Li Quan from the Tang dynasty, Meng Shi: Liang Dynasty, etc. You get eleven different ways of seeing the same text.