Darkhenge — Catherine Fisher

Rob's younger sister, Chloe, has been in a coma for three months, and his life is in disarray. To distract himself and avoid his grieving parents, Rob takes a job at a local--and mysterious--archaeological dig. There an ancient tree has been discovered, growing upside down--a tree that leads to the Unworld, the kingdom a seemingly happy and healthy Chloe presides over with no desire to return to her old life.