Here, we reach what is almost the end of Terry Pratchett's career, with one last post mortum novel due out later this year in an offshoot of a series I love.
Not that I don't love the Long Earth series, but I find it sad that we'll probably never see another book in the series, unless Baxter does what Mercedes Lackey did with Bedlam's Bard and gets a new co-author.
Anyway, once again, The Long Utopia picks up a few years after the events of The Long Mars, with The Next living in a hidden Earth known as The Grange. Lobsang and Agnes move way out in the West worlds in a community named New Springfield, where the folks keep permanent residences, but mostly step around to follow the herds. Joshua spends most of the first part of the novel looking for his father, while Sally continues to be enigmatic and under-developed.
Again, we have many disparate plots flying around, and again, they don't particularly line up that well, although they do eventually tie in at the end.
See, out in New Springfield, the current incarnation of Lobsang (now living with his adopted son under the name George) runs across a mystery the local children have discovered, involving vaguely humanoid beetle like creatures made of both organic material and metals. The Beetles seem to be able to Step "North" and "South" verses the normal "East" and "West" on the Long Earth and the Long Mars. Stepping "North" winds up in "The Planetarium", which is essentially something akin to the Delta Quadrant in Star Trek.
In the meantime, the Next are trying to recruit Stan, a Next living in Miami West 5, where they're building a space elevator that Sally's father brought back the idea for from Mars in the last book. Stan isn't particularly thrilled with the Next, who seem to be fighting amongst themselves about what essentially boils down to a "Divine right" verses a "Benevolent Despot" approach to humanity.
All of this is interspersed with Joshua's ancestor, Luis, who started off life in Victorian London performing illusions by Stepping "Widdershins" or "Diasil" (Well, to be fair, Luis starts off calling them "Dexter" and "Sinister", but the latter terms become preferred among the group Luis gets recruited to help take care of those who would work against Victoria's consort, Albert. Who isn't in a can.) Long story short, Luis and his compatriots have a bunch of adventures in stepping through World War I, and eventually set up an arrangement to marry off their descendants in perpetuity to help the Stepping Gene breed true.
So, we get a lot of thought experiments around the time we find out exactly what the Beetles are actually doing out in the High Meggers. Most of which ties in with the themes of expansion at any cost, death, and religion. Some of this revolves around the idea of creating a seed that could go colonize another planet then have that colony go off and colonize another in an ever expanding colonization process. The problem being, much like Civilization, that's unsustainable in the long run. Plus it becomes like the aliens in any number of sci-fi media, there to drain the resources of a planet, rather than coming in Peace.
And in the end, we get what I'd like to think of as Pratchett's benediction for us, his readers. A vision of his Utopia, and a strange meditation on death and sacrifice.
While I would love to see another volume in the series, where this one ends is probably where the entire thing ends, except in the hearts and minds of the readers, to whom these characters will exist in perpetuity.