It's In His Kiss (Bridgerton book 7)

Gareth St. Clair has a father who hates him and who is determined to ruin the St. Clair family fortunes. Gareth's sole heirloom is an old 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