It's in His Kiss by Julia Quinn

About the Book

All the ton agree: there is no one quite like Hyacinth Bridgerton.

Fiendishly smart, devilishly outspoken and according to some, particularly Gareth St. Clair probably best in small doses. But there’s something about her something charming and vexing that grabs one and won’t quite let go.

Gareth and Hyacinth cross paths at the annual and annually discordant Smythe Smith musicale. To Hyacinth, Gareth’s every word seems a dare, and she offers to help him out with a knotty inheritance problem he’s facing. However, as they delve into the mysterious St Clair history, they discover that the answers they seek lie not in the past but in each other; and that there is nothing as simple or as complicated as a single, perfect kiss.