Anathem is a great book, followed by a mediocre book. The great book comes early, as Stephenson builds a remarkable and convincing world surrounding a unique idea. Study and intellectualism are literally cloistered away in this world. When a person shows talents or inclination to study mathematical, philosophical, or scientific theory, that person is welcomed into a half-monastery, half-university community. That community keeps intentionally limited communication with the outside world. Although a few people of the community are allowed to talk occasionally with the outside world, most of the members stay isolated from that world, except for intervals of one, ten, one hundred, or even one thousand years. The rationale, in the society, is that technology and theory have been used in the past for remarkable destruction, and that it is better to keep such technology isolated from the greater world.
I wish Stephenson would have kept playing with this idea, and built the entire novel from it. But, after spending many, many pages laying out this convincing world, along with an entirely new set of language terms for the reader to get used to, Stephenson dispenses with it all and decides to explore an entirely different idea. I won't reveal it here, as it would constitute a spoiler, but it is significantly less interesting, at least for me. It makes all the social interactions, and the interesting world that Stephenson has built fall into the background, and that makes for a remarkably unsatisfying second half of the book. Sure, Stephenson kicks it into a high gear, with lots of ridiculously cool moments, but it just doesn't ring as true. It feels like Stephenson started with the idea of one book, got excited with the idea for another book, and rather than giving either the space that it really needs to develop, decided to write them together. It feels rather forced.
Still, a mediocre book mixed with a great book is still a good book overall. Just don't expect the volume to keep going at the same pace when you read it yourself.