Knife by R.J. Anderson.
Deep inside the great Oak lies a dying faery realm, bursting with secrets instead of magic. Long ago the faeries mysteriously lost their magic. Robbed of their powers, they have become selfish and dull-witted. Now their numbers are dwindling and their very survival is at stake.
Only one young faery—Knife—is determined to find out where her people's magic has gone and try to get it back. Unlike her sisters, Knife is fierce and independent. She's not afraid of anything—not the vicious crows, the strict Faery Queen, or the fascinating humans living nearby. But when Knife disobeys the Faery Queen and befriends a human named Paul, her quest becomes more dangerous than she realizes.