Cybernetic Walrus — Jack L. Chalker

EVERYTHING YOU THINK YOU KNOW IS WRONG...
That was the strange message left on Cory Maddox's e-mail--just at the moment when years of work on a revolutionary subspace computer system were about to pay off. Nothing would be the same for Cory again. Suddenly his life was thrown into chaos when the company that controlled his patent was sold out from under him, and instead of imminent wealth, Cory was facing immediate poverty. Then along came Alan Stark, who wanted to recruit Cory for a special research project on virtual reality.
Initially thrilled to be involved, Cory quickly discovered that there was nothing virtual about the realities he was working on. Instead, he found that Stark was on the verge of controlling the very fabric of reality itself.
Cory was unsure of Stark's ultimate goal until he began to recall pieces of another life and found himself in the middle of a battle between two groups of people who could use "rabbit holes" in space and time to jump between different realities, personalities, and lives. Whoever had control of the power to shape reality would have the power to become a god--or a devil. But before Cory could combat Stark and his minions, he first had to remember which side he was on...