The Spiral Labyrinth (Tale of Henghis Hapthorn)

It was bad enough when Henghis Hapthorn, Old Earth's foremost discriminator and die-hard empiricist, had to accept that the cosmos was shortly to rewrite its basic operating system, replacing rational cause-and-effect with detestable magic. Now he finds himself cast forward several centuries, stranded in a primitive world of contending wizards and hungry dragons, and without his magic-savvy alter ego. Worse, some entity with a will powerful enough to bend space and time is searching for him through the Nine Planes, bellowing "Bring me Apthorn!" in a voice loud enough to frighten demons.