It's in His Kiss: Hyacinth's Story (Bridgertons, Bk. 7)

Gareth St. Clair is in a bind. His father, who detests him, is determined to beggar the St. Clair estates and ruin his inheritance. Gareth's sole bequest is an old family diary, which may or may not contain the secrets of his past . . . and the key to his future. The problem is - it's written in Italian, of which Gareth speaks not a word. All the ton agreed: there was no one quite like Hyacinth Bridgerton. She's fiendishly smart, devilishly outspoken, and according to Gareth, probably best in small doses. But there's something about her - something charming and vexing - that grabs him and won't quite let go . . .