The Long Earth
by Terry Pratchett and Stephen Baxter
The Long Earth is a SF adventure in the tradition of Riverworld, Ringworld, and Missile Gap. A strange new megalithic world appears from nowhere, and everybody runs around exploring it. This is a well-trodden and quite entertaining trope in science fiction, but it is worth noting that it is not really Terry Pratchett's thing, and these books show very little of his humor and character development. I haven't read much of Stephan Baxter's works, but I would assume that this is novel was much more influenced by him.
One day an unknown inventor posts instructions on how to build an odd device on the internet. For reasons never clearly explained, a whole lot of people go out and build the mysterious machine -- a simple one, but one that does require a trip to the local Radio Shack -- and then use it. Unbeknownst to them, it is a device to shift them into a parallel universe, one in which humans never evolved on Earth. And this is repeatable ad infinitum, in either of two directions. There are now all the Earths we could ever want or need.
During the first few days hundreds of people pop off into alternate dimensions; from shock, poor choice in transition location (transporting from above the third story could well be fatal), or poor device construction, many find themselves stranded. A few people take naturally to dimensional travelling, and spend the first few days saving those less fortunate. One of the best known of these is young Joshua Valienté, although he quickly decides that he would rather run off and explore new Earths than deal with the fools on this one. He soon becomes a legend for another reason, as he is largely believed to have traveled further into the infinite expanse of Earths than any other human.
And because of this, when the first legally recognized AI person and mega-corporate mogul decides that he would like to explore the far Earths in a highly advanced uber-zeppelin, he starts by hunting down Joshua. They take off to try to see, for a start, the first million Earths. They find Earths composed entirely of water, Earths with dinosaurs, Earths with long abandoned civilizations. They find humans -- humans from their Earth -- settling much further out than they had expected, and then they start to find signs of other, modern intelligences, some of whom might not be friendly.
Meanwhile, the world economy is quickly falling apart, as there is no longer any such thing as a limited resource, land values plummet, and the population on the original Earth drops with alarming speed. A reactionary movement of humans who can't or won't travel the dimensions begins to demonize those who do, and political strife is on the rise. All in all, there is enough going on to carry the series indefinitely.
The series is based on an unfinished and long-abandoned story by Terry Pratchett called The High Meggas; for those of you who have not read Pratchett's earlier works, they are perhaps best described as 'not very good, but having interesting settings'. While The Long Earth is good, is does lack pretty much all of what makes Pratchett's later works worth reading. Aside from exploring an interesting world, there is nothing very clever in writing style here, and the characters are fairly boring and two-dimensional.
I would recommend this story to anyone who likes serious science fiction but doesn't want hard SF as much as they want well-explored fantastic science fiction, and all the more so if you like exploring truly epic universes and stories of exploring and settling new worlds.
There are at least three more books planned for the series, and the second in the series, The Long War is available now.