Darkhenge by Catherine Fisher

It's been three months since Rob's younger sister, Chloe, fell into a coma after a riding accident, and his life is in disarray. Rob's parents spend most of their time at his sister's bedside, and his best friend is afraid to talk to Rob about Chloe. To distract himself, Rob takes a job working at a secret archaeological site, where workers have uncovered a mystical ring of black timbers. At its center an ancient tree is buried upside down in the earth—a tree with the power to transport Rob to the Unworld, where Chloe lives in a forest of enchanting dreams, trapped between life and death.

Catherine Fisher has combined a fascinating exploration of myth with a modern quest for understanding. Where is the land of the imagination? And if we found our way there, would we ever want to come back?