The Water and the Wild

From the Publisher:

A green apple tree grows in the heart of Thirsby Square, and tangled up in its magical roots is the story of Lottie Fiske. For as long as Lottie can remember, the only people who seem to care about her are her best friend, Eliot, and the mysterious letter writer who sends her birthday gifts. But now strange things are happening on the island Lottie calls home, and Eliot's getting sicker, with a disease the doctors have given up trying to cure. Lottie is helpless, useless, powerless—until a door opens in the apple tree. Follow Lottie down through the roots to another world in pursuit of the impossible: a cure for the incurable, a use for the useless, and protection against the pain of loss.