The Neverending Story, by Michae Ende. ISBN 0140386335 (paperback) and ISBN 0525457585 (hardback).
A fascinating MetaBook. Besides the basic children's story, it has many subtle layers of meaning. Recommended reading.
It's difficult to find an appropriate way to describe his work: recursive and multilevel? Sure. A book for travellers... where you can find many ways and never be lost. Allegories as in a medieval book. Fantasy as in fairy tale, and much much more...