The Whisper  (English, Paperback, Starmer Aaron)

At the end of The Riverman, the first book in the Riverman Trilogy, twelve-year-old Alistair learns that his friend Fiona has been telling the truth about travelling to a magical land called Aquavania when she goes missing from their hometown of Thessaly, New York. Alistair knows her absence means that her soul has been stolen by a merciless stalker called the Riverman. Unless Alistair can travel to Aquavania to save her, Fiona may never return home. So he embarks upon a perilous journey that takes him from world to world in Aquavania, where anything is possible and the only limits are imposed by the boundaries of the imagination of each world's creator. Alistair soon learns that to find Fiona, he'll first need to confront, and possibly vanquish, the Riverman himself.