The first in a series of books by Robert Asprin
, the MYTH series
The hero of the story is actually the least heroic person you would expect to meet in this genre. This does nothing but add to the hilarity. In the story, Skeeve is studying under a powerful wizard who summons forth a demon from the dimension Perv. Just as he completes the spell, the wizard is assasinated. In the commotion, Aahz has arrived and drunk some wine laced with a magic potion that takes away his powers. The story continues as they pair decide they have to go it together surviving only on Aahz's cunning and Skeeves (lacking) magical skills.
A highly recommended read!