The Long Earth by Terry Pratchett and Stephen Baxter is a fresh take on the idea of parallel worlds. Mankind discovers the ability to 'step' into a string of alternate earths. Some people are natural steppers, like Joshua Valienté, while others have to rely on an invention called a 'stepper'. The discovery quite literally opens up endless new options for the expansion of humanity.
The idea of mankind's expansion into space is one that has been thoroughly explored (pardon the pun). Sci-fi books tend to focus outwardly on the solar system and wider universe. So I appreciate the unique approach to an oft used concept. The second book, The Long War, is already out and I look forward to reading it as well.