Forsaking All Others by Lester del Rey

A woodland spirit raised in solitude, Sylva breaks the ancient laws of her kind when she falls for a mortal man. Torn between the timeless rhythms of the forest and the messy beauty of human love, she seeks a way to bridge their worlds—even if it means surrendering her immortality. But to walk among humans is to pay a price only the gods can name, and the cost may be everything she has ever known.