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.)
The Hours and Days After: