Compare Split Infinity (Apprentice Adept #1)

On the technological, decadent world of Proton, someone was trying to destroy Stile, serf and master Gamesman. His only escape lay through a mysterious “curtain” revealed by a loving robot. Beyond the curtain lay Phaze—a world totally ruled by magic. There, his first encounter was with an amulet that turned into a demon determined to choke him to death. And there, he soon learned, his alternate self had already been murdered by sorcery, and he was due to be the next victim. “Know thyself!” the infallible Oracle told him. But first he must save himself as he shuttled between worlds. On Proton, his fate depended on winning the great Games. On Phaze, he could survive only by mastering magic. And if he used any magic at all, the werewolf and the unicorn who were his only friends were determined to kill him at once!