Thirty years ago, mankind gained access to virtually unlimited space. By means of a small box containing a potato, people could step "West" or "East" into an unknown number of alternate Earths where humankind had never evolved. Given open spaces, mankind did what mankind has always done, and colonized millions of other worlds. They weren't nearly enough.
Willis Linsay disappeared 30 years ago after releasing humanity into the Long Earth. No one knows where he's been, or what he's been looking for all that time, but now he's back and dragging his daughter along to Mars. For Mars, it turns out, also has an infinite number of alternate worlds, and one of them might just hold a whole new gateway to the universe. Back on the Long Earth, Captain Maggie Kauffman has been sent on an entirely new exploration, all the way out to Earth 200 million. Joshua Valiente, the Long Earth's oldest explorer, has set out to find a new kind of people who may be humanity's future.
The third book in the Terry Pratchett Long Earth series, The Long Mars' weakness is its plot, which feels like the set up for a bigger story. While there may be a functional double climax, most of the story is exploratory ramble, but that exploratory ramble remains absolutely stunning. Every world in the Long Earth and a few in the Long Mars developed in radically different ways. The alternate world premise may be fantasy, but every world of the Long Earth has real science behind it. Here, an entirely different evolutionary pathway, there a different sociological slant on civilization. It's possible to learn more about climate science in in a single passage of The Long Mars than an entire high school science course, and be entertained besides by Terry Pratchett's arch commentary.