Fantastic book. Just fantastic. I'm going to try to write a review that's very low in spoilers; I'm going to make another post that treats a few subjects in some more detail. Both should be generally safe, in that I'm not going to give away anything about how it ends; on the other hand I experienced a certain joy in entering this book with absolutely no foreknowledge of its contents. You might enjoy that, too.
The setting is a world called Arbre, similar to Earth in many ways; its people are human, even though its culture has gone in a different direction. In this world, there has been an enforced separation between science and technology, between theory and practice. (This is to keep technology from developing faster than culture can acclimate, though details are scarce there had been disasters far in the past that inspired this separation.)
The theors live in monastic communities, with their own system of governance independent from the outside world. The flow of information is highly restricted: some groups allow communication once a year, others once every thousand years. Little heed is paid to the rise and fall of politics and religions outside the walls.
Things begin to change drastically when an anomaly is spotted in the sky. Brilliant minds from around the world are brought together in protection of the planet. Adventure ensues.
But that's not the point of the book, really. The plot is filled out by hundreds of dialogues and thought experiments and wonderful big ideas. Everything is pseudonymous, but can be recognized as a treatment of the big ideas of Earth. (After all, truth is truth, no matter which universe you live in ... ) At no point does it come across as forced. The characters spend their lives studying their world with no tools but their own minds; the story offers a glimpse into that way of life.
And, astoundingly, there is an ending. Very few loose ends remain, except for of course one thing which is clearly necessary to be left unresolved.
I never once stopped enjoying myself while reading. I forced myself to take breaks, so that it wouldn't be gone too quickly. I think it'll take a bit of time and distance to determine if I liked it better than The Baroque Cycle, but it's certainly up there as possibly the best thing Stephenson's written.