Lore Vetinari, ruler of the city-state of Anhk-Morpork, has a problem that he must address immediately. The Postal Service in his city has become rather rundown and technological innovation seems to be leaving it behind. The Clacks, a type of visual telegraph system, allows people to send messages across great distances almost instantly, so nobody wants to use the Post Office any longer. Vetinari won't stand for this in his city, so he does what any shrewd ruler would do and offers the job of Postmaster General to a con-man who is on death row. After all, who better to breathe new life into a dying institution?
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.