It was a teenage book, fantasy, set on the moon, in which a girl goes on a quest to rescue her sister from a vampire, and ends up giving him her heart (literally and metaphorically). Does anyone remember this? It's been bothering me for years! I remember lots of detail about the book (there were helper animals including a unicorn, and a villainous Lorelei), but can I remember the book's name or author? Can I chuff!