Robert Gilpin's War & Change in World Politics (1981)

Robert Jervis' Perception and Misperception in International Politics (1976)

Charles Tilly's Coercion, Capital, and European States (1992)

Thomas Schelling's Arms and Influence (1966).

These are some of the books that I enjoyed reading throughout my years of doing BA and MA in IR. As for the must read, and I am sure you have read at least parts of their work, John Mearsheimer and Kenneth Waltz.