It's enchanting and a refreshing change from many of the books I've read in the past.
It's a clean fantasy about goblins and all types of fairies, but more so it's about a sister's love and what's she's willing to do in order to save the other. No matter how the goblin king tries to trick her or any of the other courts she's wiser to them all, until she isn't.
I won't give it way any spoilers, you'll have to read it for yourself.
If it were a movie, it'd rate PG.