Anything by Tom Holt: Expecting Someone Taller (1987), based on the mythology of Wagner's Der Ring des Nibelungen Who's Afraid of Beowulf? (1988), based on Norse mythology and history Flying Dutch (1991), based on the story of the Flying Dutchman Ye Gods! (1992), based on elements of Greek mythology including a parody of Heracles Overtime (1993), based on the legend of Blondel combined with time travel Here Comes the Sun (1993), based loosely on the Celestial Bureaucracy reinterpreted along the lines of the British civil service Grailblazers (1994), based on Arthurian romance and the quest for the Holy Grail Faust Among Equals (1994), an imagined continuation of the story of Faust Odds & Gods (1995), which features assorted pantheons and their adventures after "retirement" Djinn Rummy (1995), based on the antics of various bottle-trapped djinn along the lines of a modern Aladdin My Hero (1996), in which literary characters can move between fiction and the real world. One of the main characters is OpenDNS Paint Your Dragon (1996), based on the legend of St George slaying the dragon Open Sesame (1997), based on characters from the story of Ali Baba and the Forty Thieves Wish You Were Here (1998), in which a lake spirit grants four people their heart's desire whether they like it or not Only Human (1999), in which four human souls are switched with a machine, a painting, a lemming and a demon Snow White and the Seven Samurai (1999), based on fairy tales (Brothers Grimm and others) making a world within a computer simulation Valhalla (2000), based on Norse mythology Nothing But Blue Skies (2001), which features Chinese dragons Falling Sideways (2002) Little People (2002), based on faery legends Featuring J.W. Wells, the magic firm from The Sorcerer by Gilbert & Sullivan:
Hm, I guess Douglas Adams' Long Dark Tea Time of the Soul is too obvious, isn't it? (It's not part of the Hitchhiker's series, by the way). Or Robert Rankin?
Dark side of the sun(na) and Strata are a couple of non-DW books by Pterry that are quite entertaining, especially once you are familiar with the Discworld, they give you a bit of an insight into how things became what they were... They are SF-Fantasy rather than medieval fantasy, though...
Try the Garrett P.I. series of books by Glen Cook - Glen Cook - Wikipedia, the free encyclopedia. Humor isn't the main theme - fantasy and private investigators are - but the humor is most certainly there.
I loved reading those first three on that Glen Cook list: The Black Company (May 1984) Shadows Linger (October 1984) The White Rose (April 1985)