You know how there are some taste combinations that go well together? And there are some collaborations that produce synergistic works far better than either author has ever produced alone? Well, this isn't one of those times, I'm afraid. I have, over the years, enjoyed many books by Pratchett, and a good number by Baxter, and so I looked forward to seeing their work together, and was disappointed by what I found.
The premise is that there are literally millions of parallel Earths in existence, and that occasionally odd individuals have either had the natural ability or the ability under great duress to slip from one world to another - in a Fortean manner. When an inventor assembles a simple gadget that can be made from parts to be found at Radio Shack, with a potato for a battery, and distributes the instructions on its construction widely, the world is suddenly graced with thousands, or millions of universe-walking people, or Steppers.
And thus begins the interminable, or shall I say "Long," tale of people's travels in the universes of the Long Earth. I managed to stick it out about halfway through before sheer disinterest in the fates of any of the characters caused me to give up at last.