The Boy Who Lost Fairyland (Fairyland #4)

When a young troll named Hawthorn is stolen from Fairyland by the Red Wind, he becomes a changeling – a human boy -- in the strange city of Chicago, a place no less bizarre and magical than Fairyland when seen through trollish eyes. Left with a human family, Hawthorn struggles with his troll nature and his changeling fate. But when he turns twelve, he stumbles upon a way back home, to a Fairyland much changed from the one he remembers. Hawthorn finds himself at the center of a changeling revolution--until he comes face to face with a beautiful young Scientiste with very big, very red assistant.