Neal Stephenson has developed something of a reputation for writing novels which are both very long, and very erudite.
Anathem is something approaching an apotheosis of this - it is 937 pages long (very slightly longer than each of the individual volumes of the Baroque Cycle), and it is based, fundamentally, on themes of metaphysics, the long-term preservation of knowledge, and the nature of consciousness.
It is also a puzzle-book in many ways - the setting is a world similar to our own (to the extent that many of our philosophers and their conceptual developments exist in their history, albeit with different names), but in our relative future, where due to various world-destabilisating technologies (sorry, in the language of Abre, praxic devices) being made possible by the interaction of academics (avout), the avout have been confined to monastic cloisters, which operate on a very long time-scale. This is basically all I can tell you without giving you clues and hints about the rest of the novel - part of the point is that the reader learns the language of Arbre, and possibly brushes up on their philosophy and physics, whilst also uncovering the root of several mysteries about the world - including some which are hinted at very close to the beginning.
One hint that I will make is to note that Neal finally justifies his stance as one of the few modern SF writers who don't seem to believe in the promise of Strong AI (this was an underlying theme of The Diamond Age, and even influences Cryptonomicon, The Baroque Cycle and Snow Crash), holding that there is something Special about human consciousness.
No book exists in a conceptual vacuum, and there are clear connections to A Canticle for Leibowitz, and possibly Gene Wolfe's Book of the New Sun and Book of the Long Sun sequences. Gene Wolfe probably writes better than Stephenson, but he is also trying to do a very different thing in his novels. M Miller Jr. was concerned with some similar points - both he and Stephenson are interested in the preservation of knowledge through collapse - but Stephenson is more cynical about humanity's tendency to reject knowledge through fear and (fundamentalist) religion, while Miller Jr, being Catholic, tries to present a position that religion is necessary for morality as much as science is necessary to advance knowledge.
Definitely worth reading, if you have the time and the intellectual energy for it. If you're anything like me, though, you'll wonder why you bother trying to write anything yourself afterwards (this is the mark of a great writer for me).