Otherland - City of Golden Shadows by Tad Williams.
Set in the near future where full immersion into virtual worlds is possible, I read this series upon release in 1996, eagerly awaiting the next volume.
The simulations, some based on classic stories like Alice in wonderland, War of the Worlds and The Ilyad, are so realistic that the characters cannot discern them from reality and are in effect lost in VR.
As a geek with a keen interest in vr and the future implications of the technology it opened my mind to the possibility of misuse of vr, long before The Matrix arrived on the scene. Spanning four thick volumes it tells an epic tale which questions what reality is and how it could be manipulated for nefarious reasons.