The Long Utopia by Terry Pratchett and Stephen Baxter

Lobsang's pursuit of a human life, Joshua's search for his father and the adaptation efforts of "the Next" post-humans are challenged by a voracious alien race that would conquer and colonize the Long Earth.

"2045-2059. Human society continues to evolve on Datum Earth, its battered and weary origin planet, as the spread of humanity progresses throughout the many Earths beyond. Lobsang, now an elderly and complex AI, suffers a breakdown, and disguised as a human attempts to live a 'normal' life on one of the millions of Long Earth worlds. His old friend, Joshua, now in his fifties, searches for his father and discovers a heretofore unknown family history. And the super-intelligent post-humans known as 'the Next' continue to adapt to life among 'lesser' humans. But an alarming new challenge looms. An alien planet has somehow become 'entangled' with one of the Long Earth worlds and, as Lobsang and Joshua learn, its voracious denizens intend to capture, conquer, and colonize the new universe--the Long Earth--they have inadvertently discovered. World-building, the intersection of universes, the coexistence of diverse species, and the cosmic meaning of the Long Earth itself are among the mind-expanding themes explored in this exciting new installment of Terry Pratchett and Stephen Baxter's extraordinary Long Earth series" --

Three books in, you'd think we might have been able to dispense with the world-building activities. I confess to having grown less interested with each passing installment of the Long Earth series: The first intrigued me with the novelties of and possibilities inherent in the central conceit (people can “step” sideways into what are essentially parallel earths, with just slight differences between one and the next. Over dozens, hundreds, thousands and millions of steps, however, those differences can loom quite large). The payoff wasn't there in the first novel, but I assumed as the cosmology built out more, that feeling would dissipate.

Nope.

Instead, each new story has indulged the uge to introduce yet more “novel” mechanics and contrivances, to just straight-up skip long stretches of “insignificant” time (where there are no novel inventions propagating, and thus saving us from having to read about the “characters,” what shaped them, and other such dalliances that only get in the way off our fictional science, thank you). This is no different, where now we set upon a world where you not only can step sideways, you can also step forward and backward, and there's a Dyson sphere and ...

It's all too much. Though I thoroughly enjoyed the full third that was given over to the early history of a Victorian British secret society of steppers, its incogruity and total disconnect from the rest of the story went a long way toward proving to me that this really would be better off as a series of short stories presented from different authors (a la the Afterblight Chronicles) rather than a series of novels.

The best book in the series since the first one. Instead of just exploring the Long Earth's, here we get antagonists, real conflicts, and some interesting revelations.

We learn more about the history of the “natural steppers,” ancestors of Sally and Joshua. There's an interesting parallel between how the the British government treats the steppers and how the human governments of the Earth and Low Earths tried to treat the Next.

The big bad in this book are a group of very alien beings that the humans can't communicate with so a more extreme solution is required. Humans and Next have to work together and real sacrifices have to be made to save the future of the Long Earth.

Really satisfying. And as always the writing style makes these books a pleasure to read.