Recommended by EndersGame, it complements your World Civ 1A and 2A classes in high school. It's a collection of essays; as OSC said, it's a mixed bag. Especially informative were those on Machiavelli, Vauban, Clausewitz, Helmuth von Moltke, Delbrueck, and Revolutionary War.
Surprisingly (from what I learned in High School), Machiavelli was known in his time not for The Prince, but rather 'The Art of War', where he criticized the heavy use of mercenaries.
In 1423, in the battle of Zagonara, a victory "famous throughout all Italy, none was killed except Lodovico degli Obizzi and he, together with two of his men, was thrown from his horse and suffocated in the mud."
Unfortunately for Macchiavelli, technology advanced and it soon became necessary to have specialized troops to handle gunpowder-enabled weaponry, so mercenaries (outsourced gun technicians) lasted a bit longer. But, as technology proliferated, so did education; the newly educated and rising bourgeois class increased the technical depth of armies and the ease of command.
Vauban was given the task of defending France with a limited budget. He (apparently) went about this with great vigor, closing some bases, while refortifying (sorry) others. This architecture was the first coherent perennial national defense, and it was built on demographic studies of people, goods produced, and traversability of the region. Not bad for the 1600's.
The military import of the education of the middle class was first well-understood by the Prussians in the 19th century. Gerhard von Scharnhorst reorganized the Prussian War Ministry in 1809 and created a division with the purpose of the peacetime education and training of the army. As officers passed through the school, they received the same curriculum and were inculcated in the same culture. The vast import of this is that in battle, the chief strategist can give general directives ("take that mountain"), and the officers closer to the reality can decide how best to do this, while quickly coordinating with other officers ("you flank that side, we'll approach from the rear, as the enemy is exposed there"). Previously, generals would give very precise tactical directions, which may not be valid by the time they reached their troops. The new Prussian idea of delegating tactical decisions was called Auftragstaktik and it enabled them to kick European butt all century long.
Delbrueck was a military historian; but he was the first to take the battle as described in some other dead guy's history, and to match it up to the lay of the land, and other dead guy's histories, to see if all the facts matched. He didn't change military strategy, but he certainly changed our understanding of how battles were fought.
Finally, the essay on Revolutionary War dotes a bit much on Mao (ok, so he did manage to conquer China, sheesh), but by the 20th century we have arrived at Total War, where every single weapon that can be used, will be used. The political has fused with the military, and the general populace is now a military objective to be either subdued or preferably coopted.
Although the lessons of military history are there for everyone to read, it seems that it is the people and their institutions that determine the fate of wars. E.g. Helmuth von Moltke the Lesser (nephew of the great Prussian general)
There is a well-known story of a report reaching the German Imperial Headquarters at the height of the August crisis of 1914, indicating that the British would not enter the pending war provided that the Germans refrained from attacking France. The Emperor is said to have told Generaloberst Helmuth von Moltke (the Lesser), the chief of the general staff, that if this were true Germany should shift the focus of the offensive to the East. Moltke answered that this was impossible, because the army had only one war plan, which could not now be changed. "Your uncle would have given me a different answer," William II grumbled, but this peevish if reasonable retort did not stop the fateful westward movement of the German columns.
And even nowadays, this occurs. Contrast the success of the first Gulf War with the second. The first Gulf War had Bush, Sr. giving political strategic goals to Schwarzkopf ("Crush Iraqis in Kuwait"), who planned how to do that militarily. Contrast that with Bush, Jr, who delegated to Donald "I'm not a general, but I play one on TV" Rumsfeld, who then proceeded to tell the army how many troops it needed to secure Iraq. Maybe we all need to read a little military history.