On a clear morning in July 1804, Alexander Hamilton stepped onto a boat at the edge of the Hudson River. He was bound for a New Jersey dueling ground to settle his bitter dispute with Aaron Burr. Hamilton took just two men with him: his "second" for the duel, and Dr. David Hosack. As historian Victoria Johnson reveals in her groundbreaking biography, Hosack was one of the few points the duelists did agree on. Summoned that morning because...