Sun Tzu's Art of War is perhaps the best known and highly regarded treatise on strategy ever written. Although directly written for military strategy, this book was recommended to me by various company leaders, professors, and others.
The strategies that Sun Tzu presents can be directly applied to both entrepreneurship and leadership. There are many variations of this book - and even the shorter versions do it justice. One that I recommend: ISBN-13: 9780804830805 (it's only about 100 pages, $12, and well worth the quick read).