The series is based on the concept of parallel worlds, which can be accessed by an ability called "stepping." Each world is different in detail, and the more steps that are taken, the greater the differences become. Sapient life is rare, but natural resources are plentiful. So plentiful that survival is possible largely by simply moving along from one world to the next, hunting and gathering...which is exactly what a large portion of our world's population chooses to do.
All those parallel worlds and the impact of their discovery on our world is enough fodder for any amount of science fiction writing, but Pratchett and Baxter added plenty of other components as well. There's the super-intelligent artificial intelligence known as Lobsang, who may or may not have a reincarnated human soul. There's a major natural disaster on our home world. Humanity itself changes, as it moves out across many thousands of Earths. And of course, there are dangers out in far-away worlds that could threaten the entire continuum.
I appreciate the imagination, but I must say that it feels to me that there are more moving parts than necessary. The parallel worlds are always part of the story-line, but at times it feels like that core concept takes second place to all the other things that are going on. Having too many interesting things going on is a pretty good problem to have for a science fiction series, I suppose.
There are any number of characters used as viewpoints for the story-line. The three most prominent are Lobsang and two "natural steppers" who can easily navigate between the parallel worlds: Joshua and Sally. The character development of these and a few other major characters is fairly well done, though not in great depth. I think that lack of depth comes from the sweeping scale of this series: decades of time, thousands of worlds, and a huge cast of characters. Nevertheless, I did have a good feel for the personalities and motivations of the primary characters, even if some of the others felt one-dimensional.
I didn't feel that any one book was significantly stronger or weaker than the others. The Long Earth, first in the series, has a special place since it first introduces much of the framework of the parallel worlds. The themes of the whole series are advanced in each novel, while at the same time each of the other books has its own major focus: changes in politics, culture, and even the human species itself in the The Long War; a trip to another planet in The Long Mars; an alien menace in The Long Utopia; and expansion out into the larger universe in The Long Cosmos.
As a lover of science fiction, I found The Long Earth series to be an engrossing exploration of an imaginative world. I'd recommend it to anyone who enjoys such things.
