Touchstone, The by Edith Wharton (1862 - 1937)

Stephen Glennard's career is falling apart and he desperately needs money so that he may marry his beautiful fiancee. He happens upon an advertisement in a London magazine promising the prospect of financial gain. Glennard was once pursued by Margaret Aubyn, a famous and recently deceased author, and he still has her passionate love letters to him. Glennard removes his name from the letters and sells them, making him a fortune and building a marriage based on the betrayal of another. (Summary by Wikipedia)