PLEASE NOTE: This is neither a Regency Romance nor a mystery novel. Even though PBS lists it as a mystery, IT IS NOT A MYSTERY!!! It is a historical romance.
William the Bastard, base-born son of the Duke of Normandy, must fight the King of France to regain his Duchy. Spurned in love by the Princess Matilda, the fighting Duke dares to take a whip to her in her own fathers palace, before making her his bride. William the Conqueror sails to England to claim the Saxon Kings crown and sceptre for his own.
From an Amazon review: "Georgette Heyer, best known for her formulaic Regency and Georgian romances, strayed from her usual path long enough to turn out this excellent, entertaining novel. The narrative follows the life and achievements of William, duke of Normandy--better known as William the Conqueror--through the eyes of one of his young knights, the fictional Raoul de Harcourt."