Erh...
Now take this with a grain of salt, I'm no expert, just some guy with an interest in these things (and I own few of these books, most of what I read was in school librarys or public librarys already quite the while ago), but...
I'd say there's three categories of military "philosophy".
First, you have the "accounts of campaigns", like the "anabasis" or "commentaries on the gallic war". These texts aim first and foremost at chronicling events. They are first and foremost history books altough they will contain a certain amount of "military theory". I never get tired of pointing out how the 2 or 3 pages in the anabasis detailing the different styles of leadership are pure jewel.
Second, the true "military strategy" books like the "strategikon" (the byzantine empire, only good at discussing the sex of angels? hogwash! Even if part of the book is taken straight out of the Aelian's 2nd century works), Lidell Hart's works on tank warfare that I can't remember the name or even fringier texts like "guerilla operations". These works detail how to conduct warfare with the means of the time. These texts aim first and foremost at discussing technical details about the means and tools of the time.
Third, the "military philosophy" books. These would contain Sun Tzu's art of war, or others such as Clausewitz "on war" (if I remmember correctly my reading, aside from a few passages on the proper disposition of troops on the line of battle in the napoleonic days, most of the text deals with very broad stuff, some of it dealing more with governemental policy then military affairs per say). While the aim of these texts is to equip officers so as to develop a better understanding of their role and place in the world of man and to bestow broad guidelines and tips as to how to wield such tools as fire and espionage, their use, if you consider them side by side with more focused texts of the "second category", is more to broaden the horizons of the reader. Again, the vast number of human enterprises, from warfare to buisness management to even sports, that have found applications for the contents of "the art of war" is a testimony to the broadness and acuteness of the book.
While "the art of war" can be considered "a first", to say that every other text has built up from it is not correct. A prime example is Aelian, whom wrote quite the hefty text on ancient greek strategy but whom would surely never have been in contact with "the art of war".
War never changes...Humans never change, hence their endeavors of passion, such as war, always stem from the same glands and hormones, ambitions and needs... But war does change.
You have the huge shocks, the "revolutions", such as ameridian warfare (here I merely talk of those tribes I am familiar with of course, it's a vast world containing many demographically meak peoples of highly varying cultures) changing from ritualistic warfare with a "line of battle" and warriors arrayed with heavy wooden armors to guerilla like tactics in less then a decade following the contact with buckshot loaded firearms.
You have the smaller tweaks, such as deepening the right wing of your phalanx, which nonetheless completely upset the balance of power in ancient greece right before the macedonian domination.
But that's it: war is in constant mutation...and we need books to keep track of it. Books that, through the ages, have delt with very different subjects.
