Terry Pratchett and Stephen Baxter team up to pen this novel which has a very interesting premise: Humans figure out how to "step" which allows a person to instantly teleport to an alternate Earth. They take that concept even further by the fact that there are an infinite number of alternate Earths and one can "step" from one to the next. Since the technology required to build a "stepper" device is readily available to almost everyone on the planet this fundamentally changes civilization as we know it. Economies collapse as millions leave our Earth to go to unclaimed worlds with pristine land and resources while our existing governments struggle to deal with the concept of governing their same territory across multiple Earths.