John Tyler enters the line of presidents like a quiet hinge that suddenly bears the weight of a door no one expected to open. He is there in the background of the parade—Virginia lawyer, legislator, gentleman of the Tidewater—until the nation’s most theatrical campaign puts him in the second chair as a flourish on a slogan. Then the first chair is empty far too soon, and the man meant to be a symbol discovers he must be a system. He takes up the pen with a composure that angers those who think accidents should apologize for themselves, and he proceeds to act as if legitimacy lies in duty performed, not in the manner of one’s arrival. “His Accidency,” they sneer, inventing a nickname to reduce a constitutional question to a joke. He answers the way a careful man answers a jeer: not at all, and then with a string of decisions that make the joke ir ...