The Black Moth: A Romance of the XVIII Century by Georgette Heyer

Jack Carstares, disgraced eldest son of the Earl Wyncham, returns home in secret to live as a highwayman, the only way he can remain in the land he loves. Injured while foiling the abduction of beautiful Diana Beauleigh by the infamous Duke of Andover, Jack is taken in by Diana's Aunt Betty, and his recovery is surrounded by mystery and intrigue.