Parallel worlds is a common theme in sci-fi, but usually the focus is on alternate histories, or on what it would be like to meet a different version of yourself. I love The Long Earth because it takes the concept in a very different direction: here, the parallel Earths lack humans entirely. The multiverse is basically an enormous wilderness where anyone can have as much land and independence as they want.
The book raises fascinating questions. In such a world, where anyone could choose at any time to wander off from society and become a homesteader or a hunter-gatherer, would we be able to maintain a high-tech, highly organized civilization? If not, would that be good or bad?
Pratchett's influence is recognizable in the folksy characters and the mildly-cynical-but-fundamentally-optimistic tone. It's occasionally funny, and engaging throughout, although the secondary plot about a "humanity first" movement is developed and resolved in a way that feels sort of pointless.