Music for the Duke by Alexa Aston

Lord Henry Vaughn witnesses the deaths of his father and grandfather and becomes the new Duke of Linberry. A decent, kind, and responsible man, Henry knows after this double tragedy that it is important he take a wife and provide a ducal heir since death can come calling unexpectedly.
Lady Sophia Sawyer loses both her parents closely together and mourns them the requisite year. When her cousin, the new Lord Parkhurst, calls her to London, she believes it is so she might make her delayed come-out. Instead, Fia is banished to live in the attic like a servant, while Parkhurst schedules her to give music lessons to children of the ton, keeping the fees for himself.
The couple meets through mutual friends and the duke is drawn to the quiet, golden-haired beauty. When Fia shares she wants to play and compose her own music, however, Henry is torn, worried what Polite Society would think of a duchess who is anything but conventional.