The Long Earth is based on the idea that there are a series (possibly infinite - they pass 2,000,000 in the book) of different Earths, accessible to humans by a simple device called a "Stepper." But there are no other humans on any of these alternate worlds, so most of these Earths are almost uninhabited. And ... one small detail: you can't take Iron with you (blood is explained away because it's an Iron compound, not raw Iron). This takes an old idea (parallel Earths) and wraps it in pseudo-scientific jargon (an idea I should possibly be filing under my "Fantasy" tag rather than the "Science Fiction" one). And then tosses in a Tibetan motorcycle repairman (name of Lobsang) who's now re-incarnated as our Earth's only sentient AI. The two ideas are so jarringly unrelated that it felt like a dissonance in my head for the entire time I was reading the book. As far as I can tell, they added Lobsang as a means to travel the Long Earth quickly because they couldn't think of a better one.
I found their ideas about the politics of the Long Earth quite interesting: what happens when you have infinite plenty a "step" away, and likewise what happens when you can escape any restrictive regime or political movement by simply shifting a world over with a box made from $5 of electronics components? And what about the people who can do this without a stepper - what will they do, how will they be treated ... That part of the book was pretty interesting. But Baxter and Pratchett focussed more on the voyage of Joshua and Lobsang trying to map out the limits of the Long Earth, by going farther from "Datum Earth" (our home) than anyone else had ever done. So they spend a lot of time on creating different variants of Earth, where evolution took a different course (and sometimes physics, such as an asteroid strike). To me, it felt like we had two different novels mashed together: one grand but unthinking adventure story (Joshua and Lobsang exploring) and another thinking piece of work about the politics of the freedom of infinite worlds. The latter was the better of the two, but got less airtime.
And then, just to piss me off further, the book ended ... not with a cliffhanger, but like a chapter end in the middle of a larger book. Two of our heroes arrive at a disaster site, and someone there hands one of them a phone and says "it's for you." Literally - that's the end of the book. So several very interesting - but not exactly "science fiction" - ideas in a book with too much adventure and not enough thought. I don't think I'll be continuing with the series - there are another four books, the last two of which were published after Pratchett's death (and I'd rather be reading him that Baxter ...).