I actually have a lot of thoughts about this book, but going into most of them would require revealing more about this book’s premise than I want to in this review. So I’m going to keep this (fairly) short and highly recommend that you read the book, then maybe we can do spoilers in the comments!
The set-up of the book is that one day children suddenly start disappearing all over the world – and then hours later they reappear telling the fantastic story that they had “stepped” to another Earth, a parallel world that seems to be identical except that there are no signs of humans or human civilisation there. They were able to step thanks to eccentric scientist Willis Linsay posting online the details of how to build your own stepper device. And it turns out that there isn’t just one parallel Earth; there are hundreds, maybe thousands, maybe an infinite number, each subtly different but still clearly Earth.
“The prairie was flat, green, rich, with scattered stands of oaks. The sky above was blue as generally advertised. On the horizon there was movement, like the shadow of a cloud: a vast herd of animals on the move. There was a kind of sigh, a breathing-out. An observer standing close enough might have felt a whisper of breeze on the skin.”