Never Desire a Duke. The Neverhartts, #6

Miss Amelia Halsey is firmly on the shelf. In her first season, she'd had one offer. One she found repulsive, and thankfully her brother had refused. Now ten years later she decides it's time to give in to her one desire. She fell in love with the Duke of Darling, but he didn't notice her. At the yearly masquerade, Amelia decides she's going to try her hand at seduction, and the Duke of Darling is her gentleman of choice.
If a lady is going to have one affair it might as well be with the man of her dreams after all... Grant Barrett, the Duke of Darling, doesn't wish to marry, but thinks perhaps it's time he does. His title demands it, and so does his meddling mother. An enchanting lady with golden hair and mischievous banter lures him away from the masquerade for a tantalizing interlude. Much to his frustration she disappears when the bell sounds to remove their masks.
He believes he's finally met the woman destined to be his duchess. If only he had any way of discovering her name to ask her...