A Duke Always Has a Secret by Rebecca Leigh

The London season: when the aristocracy descended upon the city for its grand balls, garden parties, and other salacious meetings behind closed doors. Heartbroken Catherine Gray knew she would have to return to London eventually. She was the sister to a wealthy and powerful duke and would need to marry one day, but after the events of last season she feels hesitant to return to London's elite ton. After being rejected by the object of her childhood affections, she plans to prove that her infatuation has passed with age… but fate has other ideas.