Water and the wild

Lottie Fiske is a lonely twelve-year-old orphan, who lives in a boarding house, and her only friends in the world are Eliot, a boy who is very sick, and the mysterious letter-writer who sends her birthday gifts--so when a strange girl steps out of a closet and insists that Lottie follow her down the roots of the apple tree in the yard to another world, which may hold a cure for Eliot, Lottie has to go.