In the Duke's Estimation (Ducal Encounters #5)

Lady Katrina Heston has spent all her life living abroad with her diplomat father. When she is permitted to travel to London to enjoy part of the season beneath Lady Frankie St. John’s chaperonage, she’s determined to make the most of every minute before settling down to marry the man her father has chosen to be her husband.
Lord Nathaniel Sheridan, the Duke of Winchester’s youngest brother, is enchanted by Katrina’s lively personality and enters into an agreement with Lady St. John to ensure she has as much pleasure as possible whilst in the ton. He is horrified to learn that her father has agreed to throw his only child into the arms of a dullard like Brown. Convinced the scoundrel is somehow manipulating the diplomat into agreeing to the union, Nate delves deeper into Lord Heston’s affairs, with shocking results.
Can Nate save Katrina from a highly unsuitable marriage without creating a scandal that will ruin her reputation and perhaps even threaten the life of the lady he has come to adore…