Historian Anthony Pitch calls the assassination of President Lincoln "the saddest story in American history." It's hard to disagree. But there's much more to the tale than most people know. Pitch provides a fine overview of the violent forces in Civil War America in "They Have Killed Papa Dead!": The Road to Ford’s Theatre, Abraham Lincoln’s Murder, and the Rage for Vengeance. The best book about the assassin himself is American Brutus: John Wilkes Booth and the Lincoln Conspiracies by Michael W. Kauffman.
Dark Horse: The Surprise Election and Political Murder of President James A. Garfield, by Kenneth Ackerman, and Murdering McKinley: The Making of Theodore Roosevelt's America, by Eric Rauchway, are fascinating glimpses at the men who felled two obscure presidents. The best book about the death of JFK is Vincent Bugliosi's exhaustive and convincing Reclaiming History: The Assassination of President John F. Kennedy. The book's immensely readable narrative of the assassination has been spun off into a separate book. (I interviewed Bugliosi for the Monitor in 2007.)