The Riverman was one of the best books I've ever read. I couldn't believe how Aaron Starmer's mind works for him to be able to create such a brilliant world with wonderful characters. I had barely closed the first book before I was picking up The Whisper and jumping straight into Alistair's world again to find out what happened to Fiona, Charlie, Kyle and everyone else!
Before reading on, I would suggest you read The Riverman as I will mention things that happened in the previous book that you are definitely not going to want to spoil for yourselves.
We left off The Riverman with one of the craziest twists ever. Charlie, Alistair's best friend, is the Riverman, and Alistair ends up shooting Charlie's brother, Kyle, accidentally, before running off to Fiona's basement going through her portal and into Aquavania to find her. He ends up finding Fiona's world destroyed and 'swims' to other kids' worlds, meeting new people, creatures, in a quest to find her.
This is a very lonely book, following Alistair's journey as he meets a whole variety of strange characters jumping from one world to another. This is emphasized by the use of third-person in the book, whereas the The Riverman was written in first-person. I believe Starmer chose this path specifically because he wanted us to distance ourselves from Alistair, to intensify that feeling of aloneness and loneliness. We don't know what's happening in the real world, we don't know how much time has passed, or how old he is now, but we know it's a long time in Aquavania time. We also get a deeper glimpse into the history and background of Aquavania, and how The Riverman (or the Whisper) was created.
I don't know if I can honestly say this was as good as the first book, but it was different for sure. Good, but different.