Another Fine Myth — Robert Asprin

Skeeve was a magician's apprentice, until an assassin hired by an ancient enemy struck and his master was dead. Skeeve suddenly found himself alone with Aahz, a purple-tongued demon the old magician had summoned from another dimension as a practical joke just before he was killed.

Aahz had lost his powers. Skeeve had lost his job. So together, they set out through a universe populated by Deveels (or devils), Imps, dragons, unicorns and more, looking for a way to get even...