I’m going to put in a few reccomendations, but take 'em with a grain of salt: I never could get into Confederacy of Dunces, so YMMV.
Donald E. Westlake is grand fun. He has a series of books focusing on a gang of ne’er-do-well crooks, lead by a chap by the name of Dortmunder. IMO, the best of this group is Jimmy the Kid. There was a cheesy movie made from the book starring Gary Coleman and Don Adams, but it didn’t really translate well. Alas, it’s currently out of print. This collection of short stories may be a good introduction to Dortmunder and his gang, but I can’t reccomend it from personal experience.
I don’t know whether you like fantasy or not, but if you do the Enchanted Forest Chronicles by Patricia C. Wrede are very amusing. Granted, they are usually considered ‘young adult’ books, but are enjoyable for decrepit adults, too. The first book in the series, chronologically, is Dealing With Dragons, and is a good place to begin. A number of typical fairy tale and fantasy conventions get turned upside down, to great effect.
Continuing the ‘young adult’ thread I have going: Richard Peck’s books dealing with Blossom Culp are a riot. The first book in this series is The Ghost Belonged to Me, but the best book of the three I’ve read is Ghosts I Have Been.
Wodehouse is another good reccomendation. I’m inclined to the opinion that Jeeves is the most evil, and Machiavellian, servant character ever created, but that’s a minority opinion, I’ll admit. And does nothing to change the enjoyment that the books give me.
Finally, I’ll include my two favorite Farley Mowat books. He’s best known, of course, for Never Cry Wolf, which while it shows some of his humor isn’t what I’d consider a funny book as a whole. However, he has two memoires that purport to be accounts of his own experiences that are grand fun. The first is about growing up with an unusual dog: The Dog who Wouldn’t Be. The second is his account of the trials and tribulations of owning his own sailboat: The Boat who Wouldn’t Float. Both are highly reccomended.
If the touches of humor in Dog appeal to you for seeing what living with a budding naturalist might be like, I also want to suggest Gerald Durrel’s My Family and Other Animals.
And now I’ll shut up before I keep typing til sunrise.