The Changelings — Christina Soontornvat

All Izzy wants is for something interesting to happen in her sleepy little town. But her wish becomes all too real when a mysterious song floats through the woods and lures her little sister Hen into the forest...where she vanishes. A frantic search leads to a strange hole in the ground that Izzy enters. But on the other side, she discovers that the hole was not a hole, this place is not Earth, and Hen is not lost. She's been stolen away to the land of Faerie, and it's up to Izzy to bring her home.

But inside Faerie, trouble is brewing-and Izzy is in way over her head. A ragtag group of outlaw Changelings offer to help, but she must decide whether a boulder that comes to life, a girl who looks like a ghost, and a boy who is also a stag can help her save Hen before it's too late.