Otherland, book 3 : Mountain of black glass
Tad Williams1999

Trapped in the exotic virtual simulation known as Otherland, Paul Jonas, Orlando Gardner, and Renie Sulaweyo continue their separate explorations into the heart of the reality that surrounds them. As they confront puzzles and obstacles in re-creations of ancient Egypt and Homeric Greece, they come closer to the black glass mountain that may offer them the key to the mysterious Grail Brotherhood that controls the passages to and from Otherland.