Love Is Eternal: by Irving Stone.

He was from a family in log cabin and educated due to his mother's concern and care, she from a wealthy family with relatives in Illinois and Kentucky both, and they met when she was being courted by the man who would be his rival in politics as well, a man of her class and far more acceptable to her family on the whole. But Mary Todd found Abraham Lincoln far more intriguing and challenging, and in spite of his lack of social skills (recall the famous true story about her telling him that he had said he wanted to dance with her in the worst possible way) she accepted his proposal.

He went on to make history, and was assassinated soon; she did not remarry, even though it would be not merely possible but desirable to her relatives, and lived with the legacy.