Crimes, Conspiracies, and Courtship by Aubrey Wynne

Lady Matilda has always been an introvert, preferring her books to awkward conversations with strangers. As her first Season arrives, her mother insists she put away her bluestocking and concentrate on finding a husband. But Mattie is terrified of finding herself betrothed or even worse— not betrothed. The arrogant men of the ton terrify her.
Mr. Harry Walters is an orphaned, ex-Bow Street runner turned investigator, who makes a living by his wits. Working for Paddy O’Brien and his Peelers, often taking assignments for the Home Office, Walters is used to working closely with the beau monde. When a peer approaches him about a new assignment, Harry realizes they are both after the same man. He accepts the job but soon finds himself also protecting the earl’s sister.
While working in costume at a masquerade, Walters makes a fatal mistake when he asks Lady Matilda to dance. It takes only a few stolen glances and one waltz for two unlikely souls to become hopelessly entwined. Mattie is determined to win the heart of this handsome, rugged man. Harry is just as determined to keep her safe