I would suggest Clausewitz's On War. Be sure though that you are buying a book that is his complete text. There are several books out there there that are simply analysis of his works as opposed to his comlete text of On War. On the other hand, they also include writings from his many letters and military articles.
I would also include Machiavelli's Prince. It is a good compliment to Clausewitz's take on politics as war.
And then there is Patton. He did not publish any books while he lived, but there compilations of his many letters, published military orders, and diaries. One called War As I Knew It, was published by his wife a few years after the war. A much more complete work The Patton Papers (two vols.) is also available.