«Romancing Mister Bridgerton» by Julia Quinn

Penelope Featherington has secretly adored her best friend's brother for . . . well, it feels like forever. After half a lifetime of watching Colin Bridgerton from afar, she thinks she knows everything about him, until she stumbles across his deepest secret . . . and fears she doesn't know him at all.
Colin Bridgerton is tired of being thought nothing but an empty-headed charmer, tired of everyone's preoccupation with the notorious gossip columnist Lady Whistledown, who can't seem to publish an edition without mentioning him in the first paragraph. But when Colin returns to London from a trop aboard he discovers notyhing in his life is quite the same—especially Penelope Featherington! The girl haunting his dreams. But when he discovers that Penelope has secrets of her own, this elusive bachelor must decide . . . is she his biggest threat—or his promise of a happy ending?