Every schoolchild knows that John Wilkes Booth assassinated Abraham Lincoln. But history has more or less forgotten Boston Corbett, the Union soldier who helped hunt down Booth and fired the shot that killed him. As Scott Martelle discovered in researching his new book “The Madman and the Assassin: The Strange Life of Boston Corbett, the Man Who Killed John Wilkes Booth” (Chicago Review Press: 240 pp., $24.95)