The Duke's Madness (The Carroway Family #7)
Rebekah Summersby

Miss Elizabeth Dorridge was not beautiful, not refined and she had no dowry. She was not expected to make a good match; in fact she had long been thought of as a spinster. When the Duke of Harding proposed she thought all her dreams had come true. Even after he had told her of the secret in his family, the madness which would one day take him, she was amazed that he had chosen her.
In London, after they had married, Elizabeth soon learned that the Harding title was not revered as a Dukedom should be. She learned that people considered his choice in wife a sign of him impending madness, she learned that her husband secret was not so secret. Could the Duke ever love her when he feared his inevitable madness? Was there more to the secret?