Magician's Heir — D. Bruce Cotton

What if reality is the worst form of madness?
Loneliness, a failing magic business, and stacks of unpaid bills make amateur magician Adam Gray's life one of unending drudgery and desperation. He's trapped in the tiny town of Elliston, and excitement's not exactly knocking down his door.
Until it does.
A cryptic peddler, peculiar merchandise, and insatiable curiosity combine to tear Adam away from the mundane safety of his everyday existence… and into the impossible world of Tantris: a land of giants, mages and the magic Adam's hoped for his entire life. But is this a world of sanity or madness? A dream or nightmare? Adam must answer these questions, and soon.
Because Tantris has a mortal enemy -- ruthless, evil and hungry for the souls of the innocent. And the Dark Mage just chose Adam as his next victim.