The Foundations of the Science of War, by Major General J.F.C. Fuller
Fuller was a British officer during the Boer War, the First World War, and the 1920s, but he is one of the most controversial figures in the firmament of 20th century strategy. He was brilliant but arrogant, inquisitive but opinionated, and had rotten political instincts.
Yet despite all of that, he was learned in the conduct of battle and sought to create a unified theory of war in this compilation of his lectures from the British Army’s Staff College at Camberly.
Fuller never quite reaches the level of the great strategists like Sun Tzu, Clausewitz, Mahan, and Boyd, but the book serves as an excellent foil against which to pit other theories of war.