The Mythago Cycle
Book 2: Lavondyss

Young Tallis sets out on a quest to discover the forest's secrets. When she was just an infant she lost her brother Harry to Ryhope Wood. Her adolescent fancies now cause her to suspect that he is still alive - and in grave danger. Tallis follows Harry into the primal Otherworld armed only with magic, masks, and clues left by her grandfather. Eventually the primitive forest gives way to Lavondyss itself, a fascinating and terrible realm where she is forced to confront the mythagos, physical manifestations of the legends of humanity's collective unconscious.