The Folk Keeper by Franny Billingsley (1999)

A lovely, mysterious book about a young orphan girl who disguises herself as a boy and works as a Folk Keeper--one who protects the lands and its people from the deadly Folk. It's very somber, and a bit melancholy, but ultimately very moving and rewarding. Like The Secret of Roan Inish in book form.  See also Well Wished - an excellent, mysterious kids book about a well and what happened when someone wished all the children away from the village. Great, strong lead girl character and interesting fairy tale feel.