The Viscount Who Loved Me by Julia Quinn

But this time, the gossip columnists have it wrong. Anthony Bridgerton hasn’t just decided to marry—he’s even chosen a wife! The only obstacle is his intended’s older sister, Kate Sheffield—the most meddlesome woman ever to grace a London ballroom. The spirited schemer is driving Anthony mad with her determination to stop the betrothal, but when he closes his eyes at night, Kate is the woman haunting his increasingly erotic dreams…

Contrary to popular belief, Kate is quite sure that reformed rakes do not make the best husbands—and Anthony Bridgerton is the most wicked rogue of them all. Kate is determined to protect her sister—but she fears her own heart is vulnerable. And when Anthony’s lips touch hers, she’s suddenly afraid she might not be able to resist the reprehensible rake herself…