So this is the fourth book in Pratchett and Baxter's Long Earth collaboration; apparently there will also be a fifth, which feels frankly unnecessary, but there you go. I feel like the concept of the Long Earth - an apparently infinite series of alternate Earths, empty for the most part of human life, suddenly opened up to humanity - has outstayed its welcome somewhat.
Not that The Long Utopia is bad; it just feels redundant. Lobsang, superintelligent AI, has decided to retire with the reincarnated Agnes to a world deep in the Long Earth in order to settle down, raise a child, farm - to become human, in other words. But something weird is going on on Lobsang's chosen Earth, something which could threaten the whole of the Long Earth.
Why do I feel like Utopia is redundant? Well, it's not really doing anything new with the concept. Humanity has grown used to the Long Earth now, and the wide-scale changes which made the early books so fascinating are no longer happening (though the descriptions of post-Yellowstone Britain are harrowing and sad). The monster-threatening-the-whole-fandango thing was done in the very first book, to almost exactly the same formula, as was the thing where TP and SB tease us as to the revelations we might get about the Long Earth and then don't deliver on the tease. (To be honest, it feels like neither of them have - or had, in Pratchett's case *sobs* - any idea where the Long Earth came from and are just playing for time.) There's also a very boring bit in the middle set in Victorian London which has no real bearing on anything and has nothing to do with the Long Earth, which, in the absence of deep characterisation, believable dialogue or strong plot, is kind of the only reason we are here.
As I said, it's not intrinsically a bad book; it's a fun piece of writing to while away a few hours with, and the social, scientific and psychological implications of the Long Earth continue to be worked out in satisfying detail. Pratchett and Baxter's world feels real, and deeply imagined, which is no small thing in SFF. I'm just not convinced that the book was really necessary.