It sounds an interesting book, though I don't see what the plot has to do with Tam Lin.

In the ballad, a very corporeal and sexually predatory Tam Lin is held by a supernatural being. Janet meets him (on land that she owns), they have sex, she becomes pregnant and her brother tells her to get an abortion. When she tries to collect the herbs needed, an infuriated Tam Lin stops her. She learns that he is human; of equal social rank to herself; and at risk of being handed over to the devil unless she can save him, which she agrees to try to do - and succeeds.