The scene of John Wilkes Booth shooting Abraham Lincoln in Ford's Theatre is among the most vivid and indelible images in American history. The literal story of what happened on April 14, 1865, is familiar: Lincoln was killed by John Wilkes Booth, a lunatic enraged by the Union victory and the prospect of black citizenship. Yet who Booth really was--besides a killer--is less well known. The magnitude of his crime has obscured for generations a startling personal story that was integral to his motivation.

My Thoughts Be Bloody, a sweeping family saga, revives an extraordinary...