The Secret Duke

Heroine: Bella Barstowe.
Hero: the Duke of Ithorne.
It opens in 1760 in Dover, but the main action takes place in 1764.

A novel in the Malloren World.
Follows on from: The Secret Wedding.
Followed by, but is not connected to: An Unlikely Countess.

The Secret Duke was first published in Apr 2010 by Signet [9780451229533].
It reached #16 on the New York Times best seller list for Mass-Market Fiction.

More info on the book is available here: The Secret Duke