Unforgivable Duke by Anna Markland

Philip Fortescue, Marquess of Clavering is known as a hero of Waterloo. But he is also a dedicated physician working to eradicate smallpox. What people don’t know is that he is also a spy for the British Crown.
Jenny Saxton has been in love with Philip from the first moment they met. The feeling is mutual, but Philip dare not expose her to the dangers he faces in his mission to thwart an assassination attempt against the Prince Regent.
Complicating matters further is the adamant opposition of Philip’s parents to his marrying a commoner.
An old enemy resurfaces. Derrick Peploe has returned from exile in France, still determined to kill Philip’s lifelong friend, Alex, Duke of Harrowby. He sets his murderous sights on Philip when he learns the Marquess is working for Whitehall.
Readers who fell in love with these characters from Book One of the Undukes Series, will enjoy reconnecting with old friends.