I agree Robert Asprin might suit. Perhaps Lawrence Watt-Evans (The Misenchanted Sword) or Diane Duane (So You Want To Be A Wizard?) or Grant Naylor (Infinity Welcomes Careful Drivers). I also like my light-hearted books.
If you want something not necessarily sci-fi/fantasy, you might try George MacDonald Fraser's Flashman books.
M.A.S.H. and M.A.S.H Goes to Maine, the book(s) that the M.A.S.H. television series was made from by Richard Hooker were very funny. There were a lot of those MASH books written but only the first couple were written by Hooker and the others were pretty bad.
A modern contemporary fiction author that has quite a bit of humor in his writing is Richard Ford. Independence Day and The Sportswriter are excellent.