Young Minnie Sidgwick was just 12 years old when her cousin, 23-year old Edward Benson, proposed to her in 1853. Edward went on to become Archbishop of Canterbury and little Minnie--as Mary Benson--to preside over Lambeth Palace, and a social world that ranged from Tennyson and Browning to foreign royalty and Queen Victoria herself. Prime Minister William Gladstone called her "the cleverest woman in Europe." Yet Mrs. Benson's most intense relationships...