On my interview trail, I purchased this book for my Kindle DX -- and boy, am I glad that I got it electronically, since it clocks in at 960 pages and nearly 3 pounds in the hardcover edition!
It is a work of Science Fiction in the classic sense, inventing a whole world of "compossibility." A world where concents (monasteries) outlive surrounding civilizations by millenia, watching them rise and fall. The Avout (monks) revere Science, Reason, Logic and Physics instead of religion. Indeed, orders of the avout are called "maths." They strive to preserve the world's knowledge and seek out new discoveries, even though a Saunt (Saint) Lora of the 16th century proposed that all possible ideas had already been thought of, which includes Saunt Lora's own Proposition as well.
Stephenson dreamed up this world after participating in the 10,000 Year Clock of the Long Now project, where he thought of the idea of a church that had a clock with a set of individual gates, each programmed to open only at a set time: everyday, every year, every decade, every century and every millenia. A group of monks tended to the clock and were cloistered until such time that their specific gates opened.
The story follows a young "Tenner" who was collected at age 9 and finds himself released back out into the Saecular world shortly after completing his ten-year term. World-altering events call upon young Fraa Erasmus and his fellow clock-winders to solve a great mystery and save Arbre.
Stephenson definitely does not cut you any slack in the novel as you are introduced to the "Orth" language of the people Arbre on the first page and he never lets up. Slowly, the definitions for all of these words unfold in the novel. The payoff is rich in the form of contemplative prose in classic Plato and Socrates style dialoging that encompasses the physics of geometric coordinate phase space, the philosophy of consciousness, the existence/non-existence of time, the power of observation, and the exciting possibilities of science fiction (yes, there are parts of the book that include classic sci-fi space-opera elements to make geeks and nerds glomph and squeal and huggle and hurr.)
If you enjoy xkcd comics, Snow Crash, His Dark Materials, or anything along this vein where education itself is a form of entertainment for you, then this book is a definite buy.
It is quite a struggle for me to describe how this book has affected me, save to say that I have some upsight into the possible tracings of my own Narrative along multiple Worldtracks as they progress along a Directed Acyclic Graph in a Wick fashion, leaking in from the Hylean Theoric World. Complete nonsense to most, but I could put it best in no other way.
And that's Not My Second Opinion.