I dare say I was a bit disappointed by this novel, presumably the last in the Long Earth series.
This book picks up a couple of decades after the last novel and as such all of our favorite characters are much older and a bit more jaded than they were before. There is talk of a new expedition interleaved with some kind of "message" that people up and down the Long Earth are receiving, but it's frankly never explorer to my satisfaction. I just felt the book was more filler than substance, and perhaps fleshed out in large from by Stephen Baxter.
This is an okay book and if you're a fan of seeing how the series wraps up it's worth getting. It's simply not as interesting or as filled with "new ideas" as the previous books were. I don't really see how there will any more in the series unless they pick up the pace a bit, but we'll see.