Simulacron-3

A virtual-reality novel from a time before virtual reality, Simulacron-3 is a prophetic tale of a future where nothing is as it appears to be.
Douglas Hall is part of a team that builds an artificial environment to simulate reality. This enables them to get public opinion polls without waiting for the opinions of people around them. But then something goes terribly wrong and his partners on the program start disappearing.
But is it a simulated disappearance, or is someone out to get them all? And what is the true nature of reality?
Stories based on Simulacron-3 have been adapted for both television and movies, and the book is considered a favorite of many of the masters of science fiction.