Geoffrey Best, War and Law Since 1945, (Oxford: Oxford University Press, 1994) A rather dry but very thorough exposition of the treaties which have created the international law of war.
Michael Bilton and Kevin Sim, Four Hours in My Lai, (New York: Penguin, 1992). A history of America's worst recent moment of murder, jingoism, and hypocrisy.
J.L. Brierly, The Law of Nations, 6th Ed. (Oxford: Oxford University Press, 1963). The classic summary of international law.
Robert E. Conot, Justice at Nuremberg, New York: Carroll and Graf, 1983) A straightforward, workmanlike history of the first Nuremberg trial.
Alain Destexhe, Rwanda and Genocide in the Twentieth Century (New York: NYU Press, 1994). A history of the Rwandan massacres, placed in the context of 20th century genocides.
Telford Taylor, The Anatomy of the Nuremberg Trials, (Boston: Little Brown, 1992). A history and reminiscence by one of the American prosecuting attorneys.
Committee on Foreign and International Law, New York County Lawyers' Association, "A Comparison of the Yugoslavian and Rwandan war Crimes Tribunals" (report of November 13, 1995)
The Coalition for International Justice supports the Bosnia and Rwanda tribunals and has lots of source material online.
International Human Rights Instruments collects most of the treaties that have laid the basis for international law.
CourtTV has collected some background information on the Serbian trial, which it is planning to televise.