The Touchstone by Edith Wharton audiobook.
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.