Otherland

From Wikipedia: "Otherland is a series of four science fiction novels written by Tad Williams. The story is set in the mid-to-late 21st century (Some hints to a character's age shows that the story takes place in the 2070s) where technology has advanced somewhat from the modern day. The most notable advancement has been the development of the ability for people to attach themselves fully to a computer via a nerve interface in order to experience an online world, called simply “the net”, as virtually real. Tad Williams weaves an intricate plot spanning four thick volumes and creates a picture of a future society where virtual worlds are fully integrated into everyday life."

The four books are:
1. City of Golden Shadow
2. River of Blue Fire
3. Mountain of Black Glass
4. Sea of Silver Light