As a fan of Pratchett, Tolkien, Mr Adams, Asprin etc I can happily recommend
Seemingly unheard of trilogy by Dan McGirt- Jason Cosmo
http://jasoncosmo.pbworks.com/ one of teh funniest! easily on a par with Asprin but he has only done 3 books! 20 years ago!!!
Also Craig Shaw Gardener is pretty good his malady of magiks series.
Alan Dean Foster of course maybe not laugh out loud but definitely good humour in his spellsinger series and Quozl and many of his older non movie based books.
And my final recommendation for a series i loved are the Cineverse cycle, couldn't remember the author but its also Craig Shaw Gardner! these are one of my favourites too very cheesy and very pratchett. about a guy transported to a world where B movie rules are in place