(It appears that I speak at greater length when angered than when pleased.)
John Percival Hackworth, a brilliant nanoengineer in a world in which nanotechnology has eliminated scarcity, is hired by an eccentric aristocrat to create an AI book - really, a video game - to teach his spoiled granddaughter to be self-reliant and unconventional. The book is titled The Young Lady's Illustrated Primer.
Hackworth, inspired by the project and determined to give his own young daughter the same benefits, has an illicit copy of the Primer made, only to have it stolen. It falls into the hands of an abused, illiterate, and brilliant little girl named Nell. Dr. X, the black market kingpin who made the copy for Hackworth, has an interest in the Primer for his own purposes, and Hackworth finds himself being blackmailed. As Nell learns from the Primer, other people develop an interest in Nell, including a voice actress Miranda, who does the voice of the book and unwittingly becomes Nell's surrogate mother.
That's the plot, but the plot is second to the worldbuilding - which, unfortunately, is very unpleasant. The book's ideas about what cheap nanotechnology might do to the world are fascinating, but they come along with a lot of other ideas, which are interesting only the sense that they are bad.
Neal Stephenson's revulsion for society makes it difficult for him to distinguish one of them from another, and most of the cultures he describes are the same dystopia. He gives the impression of having read some social psychology, and come out of it feeling the sort of humiliated bitterness that a teenager raised Young Earth Creationist might get from bio class. How dare they lie to him and tell him he's an individual! It turns out that a human being's social environment affects his or her behavior!
This is an obvious contradiction!( Cut for length; spoilers are under selection-tags. )