Number One on his list:
Kenneth Waltz, Man, the State, and War.Read about the other nine titles on Walt's list.
An all-time classic, which I first read as a college sophomore. Not only did M, S & W provide an enduring typology of different theories of war (i.e., locating them either in the nature of man, the characteristics of states, or the anarchic international system), but Waltz offers incisive critiques of these three "images" (aka "levels of analysis.") Finding out that this book began life as Waltz's doctoral dissertation was a humbling moment in my own graduate career.