Book 48 The Long Mars by Stephen Baxter and Terry Pratchett
I think this is the last of this partnership - I'm sure I read there were only going to be three books in the Long Earth series. The story follows several different threads as humanity is still exploring the new reality of the 'Long Earth' - different versions of Earth which can be reached either by using a box called a stepper to shift you from one to another, or by the rare few humans with a natural ability to step between worlds. There is the exploration itself, as two craft called twains are sent out further into the parallel worlds than any others have been before. At the same time, a small expedition is finding out that it is possible to step between different versions of Mars, reached through the Gap, a universe where Earth had been destroyed. Then there is the discovery of a new type of human, far more intelligent and in many ways 'other'.
As I said to Remus, I think the middle book of the trilogy was probably the weakest. This one I found intriguing for the diversity of the threads, as well as the myriad worlds created by Baxter and Pratchett. At heart, though, it is an exploration of humanity as much as an exploration of imaginary worlds. Good book.