I've also been reading Robert Asprin. So far I've read Myth Conceptions, Another Fine Myth, Myth Directions and Hit or Myth. They're candy, really. Very light reading. Appropriate for middle to high school age readers. At least where I think that age level should be reading. Most of the kids at that age in my neighborhood would think these books too thick. They'd fall just below the Xanth series of books.
What we have is a magician training his apprentice. The apprentice wants to be a thief. To prove that the training is going somewhere the magician summons a demon (a.k.a. dimension traveler). The "demon" is a magician from another dimension. Both magicians like to summon each other to show off their skills. They also like to play pranks on each other. The summoning magician, in this case, has stripped the demon of his magical abilities.
Then an assassin comes in and kills the summoning magician.
So the demon take the apprentice under his wing and they go off to stop the wizard who ordered the hit and save the world.
So that's the basic premise of all the books. The apprentice and the magician with no powers getting themselves into all kinds of goofy situations and conning their way back out.