The Long Earth isn't just a story about parallel universes; it very nearly qualifies as an artifact brought over from one. It is the first book in a series that an alternate Pratchett elsewhere in the multiverse had time to write because he wasn't busy writing about the Discworld. From what I can gather, Pratchett was working on it in 1986 and, had 
 The Light Fantastic 
 proved less successful, The Long Earth would likely have been published instead of 
 Equal Rites 
 . Sir Terry's friend Stephen Baxter got hold of the mouldering rough draft a couple of years ago and thought he might do something with it. 
 I love the premise of The Long Earth, there's wonderful potential in the ideas here, but the book was somewhat unsatisfying. I have two main complaints: first, the whole book is spent setting things up (as if it were the first episode of a TV show); second, there are occasional jarring stylistic shifts between the hard SF elements and lighter, humorous moments. The basic premise is a wonderful, open-ended one which could easily support a series as long-running as Discworld has been. But the Discworld books all function as stand-alone novels; I continue to read them, after all these years, because I always find that I enjoyed the previous one so much. I understand that a Long Earth sequel is in the works. I'll read it, but only because I want to know what happens next.