4 1/2 stars.
The Long Earth is the first of a planned trilogy by Terry Pratchett and Stephen Baxter. If you were looking for two of the most unlikely authors to collaborate, you'd be hard pressed to choose better candidates than these.
Pratchett, as pretty much the entire world knows, predominantly writes humorous fantasy, and while it's true that his work has evolved from its beginnings as pure humor to take a much deeper, more profound look at the world through the medium of fantasy, his major appeal is still the humor.
Baxter, on the other hand, is the hardest of hard science fiction authors. His books are meticulously researched, and his speculation is firmly rooted in bleeding edge science. Like Pratchett, Baxter has evolved, in his case to include more believable, rounded characters with real stories. But when you approach a Baxter book you do so for the science fiction. (Even in his alternate history Northland series, Baxter follows the logic of his premise with a sharp, unyielding, scientific focus.)
If you approach The Long Earth expecting to find something matching either Pratchett's or Baxter's usual output, you are going to be coming at it all wrong. This is a genuine collaboration, and between them they have produced something quite different from their normal works.
In the year 2015, mankind suddenly discovers the existence of possibly infinite alternate worlds, differing only marginally (but progressively, the further out they are) from our own, which can be reached by the means of an electronic device that anyone can easily assemble. But there is one thing that is different about all of these worlds: humanity hasn't evolved on any of them.
The Long Earth explores the consequences of this discovery, and follows the exploration by Joshua Valienté, a "natural stepper", who can cross rapidly between worlds without aid of a device, and Lobsang, an AI who claims to be the reincarnation of a Tibetan motorcycle repairman.
The thing The Long Earth most reminded me of was Philip José Farmer's Riverworld, with its exploration of the unknown, mysterious new world the characters now find themselves in, and the overarching questions of what it all means and what it's for. And that's pretty good company for the book to find itself in.
There are one or two places where it seemed clear to me that either Pratchett or Baxter was responsible for a passage, but remarkably, in most of the book, you really couldn't tell, and that's a pretty impressive achievement for two such distinctive writers.
Most of the criticism I've seen about this book seems to come down to people expecting to read something just like Discworld and then being unhappy that it wasn't. It isn't supposed to be. It's very much its own book, and it's all the better for it.
Expect imaginative, accessible science fiction with a sense of wonder and a light touch, and that's exactly what you'll get in The Long Earth.