Beware Faery Gifts — M.J. Putney

A shy mortal girl rashly accepts a faery lord's offer of beauty and charm. Then he comes to claim a terrible price…

Quiet young Leah lives a lonely life, her greatest pleasure being to play her Celtic harp in the woods. Her music attracts the attention of Ranulf, a faery lord who yearns to claim Leah and her music, so he offers her beauty, charm, and a season in London where she can find a grand husband. When the time comes to pay him back, he'll offer three choices, and surely one of them will be easy…

The faery lord's gifts gave Leah the promised beauty and charm and her godmother unexpectedly offers to sponsor a season in London, where she falls in love with Duncan, a handsome, magnetic hero of Waterloo. But a dazzling faery lady from India has also taken an interest in the affairs of the humans, and when it's time for Leah to pay her debt to the faery lord, she faces an impossible choice.