Stephen Walt's Ten Best Books in IR

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1). Kenneth Waltz, Man, the State, and War.
2). Jared Diamond, Guns, Germs, and Steel.
3). Thomas Schelling, Arms and Influence.  
4). James Scott, Seeing Like a State: How Certain Schemes to Improve the Human Condition Have Failed. 
5). David Halberstam, The Best and the Brightest. 
6). Robert Jervis, Perception and Misperception in International Politics.
7). John J. Mearsheimer, The Tragedy of Great Power Politics.
8). Ernst Gellner, Nations and Nationalism.
9). Henry A. Kissinger, White House Years & Years of Upheaval.
10). Karl Polanyi, The Great Transformation. 

So that's ten, but I can't resist tossing in a few others in passing: Geoffrey Blainey The Causes of War; Douglas North, Structure and Change in Economic History; Valerie Hudson and Andrea den Boer, Bare Branches: The Security Implications of Asia’s Surplus Male Population; Robert Gilpin, The Political Economy of International Relations; Steve Coll, Ghost Wars; T.C.W. Blanning, The Origins of the French Revolutionary Wars; R. R. Palmer, The Age of the Democratic Revolution; Avi Shlaim, The Iron Wall: Israel and the Arab World; Stephen Van Evera, Causes of War; Samuel Huntington, Political Order in Changing Societies; Tony Smith, The Problem of Imperlalism; and Philip Knightley's The First Casualty: The War Correspondent as Hero, Propagandist, and Myth-Maker. And as I said, this just scratches the surface.    

Source: Stephen Walt, "My "Top Ten" Books Every Student of International Relations Should Read,"
Foreign Policy, April 9, 2009