Anathem by Neal Stephenson
In the beginning of this novel, there is a Note to the Reader, the first line of which is:
“If you are accustomed to reading works of speculative fiction and enjoy puzzling things out on your own, skip this Note.”
So I did. And my first recommendation to anyone thinking of reading this book is to learn as little as you can about it, including skipping that note.
The second recommendation to anyone thinking of reading this book is to ignore this xkcd comic:

The comic is either dissing Anathem, or warning it, not sure which. If you go to the xkcd website and mouse over the image, the text that pops up says “Except for anything by Lewis Carroll or Tolkien, you get five made-up words per story. I’m looking at you, Anathem.”
So, we know to discredit this particular comment because a) he allows an exception for Tolkien, who is boring, and b) he’s dissin’ Stephenson. Or threatening Stephenson that the novel better not suck whenever the comic artist gets around to reading it. Not sure which.
Regardless, I actually agree with the Rule of Thumb, I would just replace Tolkien with Stephenson in his list of exceptions. I’m indifferent on Lewis Carroll. (If you’d like to debate exceptions, I direct you to the xkcd forum, where there’s somthing like 10 jillion posts about this comic debating that very topic).
I had trepidations when I read a blurb about Anathem when it first came out. Too many speculative fiction books “create” a world by just making up different names for stuff, and Stephenson makes up a hell of a lot of words in the blurb alone.
But, as another commenter put it, the first part of the book is a pretty impressive bit of world building. It only took a few pages before the multitudes of made-up words clicked and I stopped noticing them or caring.
As to the ending, to be perfectly honest I’m going to liken it to watching the movie Primer, a low budget but extremely good time travel movie1. I really enjoyed it, but I was hanging onto comprehension by my fingernails. I may need to go back and read the last 100 pages again, just to make sure I followed what happened correctly. In my own defense, I was really into it at this point and taking every opportunity to read a few pages. So I’d get it in little 5 page bursts, interrupted by a kid or dog or kid-dog related emergency. Theoretically, I should have just waited until I had a block of time, but… you know, Stephenson. I’m a fan.
I would bet $1,000 that Stephenson has read Walter M. Miller’s A Canticle for Leibowitz.
Some folks are saying that Anathem is better than Cryptonomicon. My vote is still with Cryptonomicon and, for that matter, The Baroque Cycle trilogy. But Anathem is a close second (fifth?), and tied with The Diamond Age: Or, a Young Lady’s Illustrated Primer, and just ahead of Snow Crash.
Glad I could clear that up for everybody.
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1 Which is not to say that Anathem is a book about time travel. I’m not saying what it’s about. This log is spoiler free, more or less. I’m just saying that what it’s about is complex, like time travel narratives can be.