Here's some suggestions:
James H. Billington, Fire in the Minds of Men (traces the rise of Communism from the French Revolution)
Laurence Lafore, The Long Fuse: An Interpretation of the Origins of World War I
Bernadotte E. Schmitt and Harold C. Vedeler, The World in the Crucible: 1914-1919
Sally Marks, The Illusion of Peace: International Relations in Europe, 1918-1933
John Keegan's recommended reading list for World War II (to which I'd add, if you like military history: John Pimlott, The Historical Atlas of World War II, and Stephen Badsley (ed.), Atlas of World War II Battle Plans; Eric Larrabee, Commander in Chief, a great study of the Allied war plan; and anything by William B. Breuer who is one of my favorite historical writers on WWII and the Cold War)
Martin Walker, The Cold War
Post-Civil War America:
Frank W. Klingberg (ed.), A History of the United States: from 1865 to the Present
Walter Isaacson and Evan Thomas, The Wise Men (a biography/history of six key US foreign policy advisors from WWII to the Vietnam War)
John Earl Haynes and Harvey Klehr, Venona: Decoding Soviet Espionage in America
Steven M. Gillon, Politics and Vision: The ADA and American Liberalism, 1947-1985 (traces the Democratic Party's internal conflict between radicals and moderates)
Len Colodny and Robert Gettlin, Silent Coup: The Removal of a President (on Watergate)
Heh...I'm not going to be gone that long...
But thanks for the list. I'll print it out and keep it for reference.