Book one, Trylle trilogy. Author: Amanda Hocking.

When Wendy Everly was six-years-old, her mother was convinced she was a monster and tried to kill her. It isn't until eleven years later that Wendy discovers her mother might have been right. 

With the help of Finn Holmes, Wendy finds herself in a world she never knew existed - a world both beautiful and frightening, and Wendy's not sure she wants to be a part of it.