Kin (The Good Neighbors, #1)

Rue Silver's mother has disappeared . . . and her father has been arrested, suspected of killing her. But it's not as straightforward as that. Because Rue is a faerie, like her mother was. And her father didn't kill her mother -- instead, he broke a promise to Rue's faerie king grandfather, which caused Rue's mother to be flung back to the faerie world. Now Rue must go to save her -- and must also defeat a dark faerie that threatens our very mortal world.