by Terry Pratchett & Stephen Baxter
This, the third book in the ongoing series, expands on the future of humanity. It's now the mid-21st century, a couple dozen years after Humanity discovers a means to travel to parallel Earths. All are empty of intelligent life, so humans spread and settle onto thousands of worlds with plenty of space and plenty of resources. Nations spread their influence over their equivalent lands. But some worlds are dead, barren of life. If that's true, is there a Mars that does host life?
Figure out how we got got in the first two books, The Long Earth and The Long War.