Heart’s Desire by Wendy LaCapra

Lady Clarissa has decided to live life on her terms. After the end of a ten-year betrothal, she wants nothing to do with marriage or the men of the ton. Least of all her friend’s brother, the very charming Lord Markham, or Hearts, as many ladies call the oh-so-handsome earl.
Markham pursues relationships with no ties that bind. Acting the rake leaves everyone satisfied…until he overhears a wager that could lead to Clarissa’s ruin. He can’t help but step in and claim she’s his intended bride.
Clarissa is appalled. She did not need to be saved. Reluctantly, she agrees to the fake courtship, if only to experience what the rakish Markham can offer. But when lust becomes love, Clarissa must make up her own terms and bet it all on Hearts.