Stephen Baxter is one of the best science fiction series written this decade. The story takes place in modern days on a planet earth where the population has recently discovered how to pass into parallel realities. More interesting than the discovery of alternate dimensions is the lack of humans in any of the other infinite dimensions. Humans, upon learning that there are empty worlds with unclaimed land and no government, start to settle on parallel earths in a migration that is akin to the Western expansion of the United States under manifest destiny.
Joshua, a natural stepper who can travel between worlds without the use of any technology, and Lobsang, a motorcycle repairman reincarnated into a computer, decide to travel the full extent of the infinite dimensions together, in hopes of finding what lays in the farthest reaches of the worlds. What they find on those parallel earths is very different from anything they were expecting.
I would suggest this title for anyone who enjoys character development and action that keeps coming. I would also suggest it for fans of classic science fiction, Lewis and Clark, and Westerns.