The Whisper: The Riverman Trilogy, Book II

Twelve-year-old Alistair Cleary has washed up on shore in what seems to be Aquavania, the magical realm where children create entire worlds from their imagination. There's something wrong, though. The creators have disappeared and the worlds are falling apart. All Alistair wants is to find his friend Fiona Loomis and go home. Easier said than done. To find Fiona, Alistair must travel from world to world, confront the mistakes of his past, and he must face countless monsters, including the soul-stealing stalker that some people call the Riverman, the merciless but misunderstood servant of Aquavania who refers to himself as the Whisper.