The Long Earth by Terry Pratchett, Stephen Baxter
This novel explores the concept of an infinite series of parallel worlds, each differing slightly from our own Earth, which become accessible to humanity through a simple, homemade device. The story follows the adventures of Joshua Valienté, who is naturally able to step between these worlds without the need for technology, and Lobsang, an artificial intelligence claiming to be the reincarnation of a Tibetan motorcycle repairman. Together, they embark on an epic journey to explore the far reaches of the Long Earth, discovering new forms of life, human settlements that have adapted to these unique environments, and the profound implications of this endless frontier on human society and its future.