I said lots of other clever things that I can't remember now, but here's my conclusion:
The plot is original but has the inevitable feeling of a folktale: the orphan finds a home; the child discoveres her true identity. Corinna's journey into herself is surprising, convincing and satisfying. I found The Folk Keeper atmospheric and suspenseful, and quite, quite unique. (I think I may not wait for our library to get Chime; I don't usually buy books before I read them, but I feel pretty confident about Franny Billingsley as an author.)
The Folk Keeper is a salad of wild greens and bitter herbs with a few curls of Grana Padano cheese and a very light, lemony dressing, served with artisan bread and fresh butter.