Heart’s Desire by Wendy LaCapra

After the end of a ten-year betrothal, Lady Clarissa 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 the oh-so-handsome earl is called. 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. Reluctantly, Clarissa agrees to Markham's fake courtship, if only to experience what the rakish Markham can offer. But when lust becomes love, Clarissa must decide if she can bet her all on Hearts.