Used that database I mentioned earlier. I looked up Terry Pratchett and it generated the following as similar authors:
Jasper Fforde
Neil Gaiman
Spider Robinson
Plugged in Neil Gaiman and got:
Terry Pratchett (duh)
Alan Moore
Robin McKinley
Kurt Vonnegut
Stephen King
Generated list of Fantasy humor (due to my lazy copy/paste, the author's last name is first):
The golden dream of Carlo Chuchio by Alexander, Lloyd
Swords for hire by Allen, Will
Once upon a Marigold by Ferris, Jean
The princess bride by Goldman, William
Just Ella by Haddix, Margaret Peterson
Dark Lord of Derkholm by Jones, Diana Wynne
Howl's moving castle by Jones, Diana Wynne
Goose chase by Kindl, Patrice
Owl in love by Kindl, Patrice
Wizards of the game by Lubar, David
Bras & broomsticks by Mlynowski, Sarah
The wizard, the witch, & two girls from Jersey by Papademetriou, Lisa
Fruits basket, vol. 1 by Takaya, Natsuki
The shadow thieves by Ursu, Anne
Never trust a dead man by Vande Velde, Vivian
Flora Segunda by Wilce, Ysabeau S.
Dealing with dragons by Wrede, Patricia C.