First, a word of caution: I bought the 1100-odd page mass market paperback of Cryptonomicon, and found that as an edition, it was a little tougher to read: no margins to speak of, and a bit of an odd shape for a novel… perhaps borrow a more expensive edition from a library, and you’ll have a better time of it.
It’s a tale of cryptography told, in different chapters between the second world war and the present day, but all intertwined. This cyberspace tome has aged well (apart from the references to “floppy disks” – does anyone use them anymore?), and remains an engaging, and clever story.
There are parts of the story that are gross, others are thoroughly nerdy, though always in an approachable way: I don’t know a great deal about cryptography, but I have more idea than most, I would imagine.
For more detail, Karen’s review of Cryptonomicon offers something by way of summary.