Carl von Clausewitz - On War would probably be a good place to start.

There's an online version here: http://www.clausewitz.com/readings/OnWar1873/TOC.htm

Clausewitz was 19th century. Find a copy of Frederick the Great's thoughts on warfare. I recommend Jay Luvaas's "Frederick the Great on the Art of War". It's a pretty good discussion of how he wanted war to operate in theory, though that did not always happen in reality. To see him placed in context, check out Parker's "Cambridge History of Warfare".

I would recommend Russel Weigley's The Age of Battle: The Pursuit of Decisive Warfare which goes through the tactics and strategy of the Early Modern Era.