The Long Earth, from
The Long Utopia, by Terry Pratchett and Stephen Baxter.
It's been the best-drawn world I can remember reading for a long, long time: it's been extensively thought through, and I always find it hard to believe that it's not real (which, considering that it's pretty far beyond the realms of possibility - being a chain of worlds that humans can "step" to using a potato and some basic electrical equipment - is not an achievement to be sneezed at).