Almost everything by Terry Pratchett. Just beware the last few books, where he was clearly suffering from Alzheimers. "Good Omens" (with Neil Gaiman) is a fantastic starting point. In the laster books, Pratchett was using his fantasy setting to dissect current social problems, usually very effectively; the 'boot theory' comes directly from Pratchett.

Everything by Douglas Adams.

Charles Stross' "The Laundry Files" series; how bureaucrats deal with Lovecraftian horrors.

Glen Cook's "Garrett P.I." series. Not really comedy, but has a strong element of the ridiculous, and plays off of the tropes established by Dashiell Hammett and Raymond Chandler in a high fantasy setting.