"Neuromancer"/"Count Zero"/"Mona Lisa Overdrive" by William Gibson (1984, 1986, 1989)
The dystopic sci-fi trilogy conceived of online culture back when the Internet was still the province of well-connected science and government types, and described the medium through which it would happen. As Gibson explained it, cyberspace was "a consensual hallucination experienced daily by billions of legitimate operators, in every nation, by children being taught mathematical concepts.... A graphic representation of data abstracted from the banks of every computer in the human system. Unthinkable complexity. Lines of light ranged in the nonspace of the mind, clusters, and constellations of data."