The Secret Duke (The Carroway Family #4)
Rebekah Summersby

Miss Sarah Mathon had a desire to have financial stability, she was an orphan and had been relying on her extended family for everything ever since. She wanted her own home, her own family, and an ability to clothe herself without feeling guilty for how much it costed her extended family. She needed to marry a wealthy gentleman. She found him in the Duke of Harding, who was titled and wealthy, but who owned a secret.
For Sarah, Mr Phillip Contel was a bad distraction since he was a younger son and had no wealth, but the attraction she felt for him was sometimes more than she could ignore...
Phillip knew without a doubt that he would marry Sarah, but when she announced her engagement to the Duke of Harding, suddenly he doubted...