A richly detailed book coming in at roughly 900 pages (plus an extensive glossary and several "calcas" or lessons), Anathem by Neal Stephenson, is not a lightweight read in either the physical or mental sense of that phrase. This hefty book will challenge you.
Set in an alternate "cosmi" (universe), the world of Arbre is divided between secular powers and a complicated system of "concents" (monasteries) devoted to the study of scientific and philosophical truths. Although borrowing heavily from the milieu of medieval monasticism, the monks here are, for the most part, not religious. They're closer in attitude to the thinkers of ancient Greece than the more familiar devout who worshipped God and (as a bonus) preserved western thought and knowledge during our own dark ages.
I spent the first quarter of Anathem trying to sort out a myriad of monastic orders, exotic names, and philosophies of truth and science. There's a lot to keep track of and I didn't click into the book right away. But once I did, I found that I was in for a stimulating ride.
There is a basic and workable story at the heart of Anathem (visitors from another cosmi wreak social and military havoc on Arbre), but the real meat of the book comes from numerous and lengthy discussions of on the nature of existence.
This may not sound very action-packed (it isn't), but it is quite interesting and stimulating in that it makes you consider topics that you probably haven't thought about since staying up way too late in college.
If you're looking for the SF equivalent of a beach read, move on. If you want to be challenged and get your mental gears turning again, read Anathem.