The killer plays a smaller role in "'They Have Killed Papa Dead!': The Road to Ford’s Theatre, Abraham Lincoln’s Murder, and the Rage for Vengeance" by Anthony S. Pitch, who calls the assassination "the saddest story in American history." It's a bold claim, but he provides plenty of support in this 2009 book.
Pitch tracks the hatred of Lincoln back to 1861, when the president had to sneak into Washington to avoid an assassination plot. He chronicles the devastating impact of the assassination on Lincoln's family and the cruel injustices that came after the president's death.
(By the way, Pitch offers a Lincoln assassination walking tour to Washington D.C. visitors, taking them to several sites mentioned in the book. I took the tour last year and highly recommend it.)