Authors that have made me laugh out loud reading on public transport:
Canadian Mordecai Richler, banned South African now Englishman Tom Sharpe, Philip Roth - The Great American Novel got me into baseball, all of Bill Bryson is laugh out loud funny, Carl Hiassen if you like thrillers or maybe Kinky Friedman.
Bridge of Birds by Barry Hughart, a chronicle of the adventures of Master Li and Number Ten Ox in a fantastic version of ancient China. I have read it at least once a year for many years now, and I still laugh out loud at it. It also has the virtue of engaging characters and a rather beautiful story. I believe it’s available in an omnibus with two other books of their adventures (the remainder of the chronicles having been destroyed by the Neo-Confucians for corrupting the Empire, as I recall ;)).
Another slightly odd pick: Calamity Trail by Dan Parkinson. It’s hard to find, being out of print, but you might find a cheap copy on Amazon. I’m not into westerns, and would never have read it had I not heard my father–who almost never laughs out loud–snickering over it. It turns out that it’s more surrealist slapstick comedy wrapped around fantasy wrapped around a parody of a western.
Thorne Smith wrote some great comic novels back in the 20s & 30s - he’s maybe best remembered for Topper but I’d suggest either Turnabout or The Stray Lamb. In one a husband and wife switch bodies and in the other a man turns into a series of different animals.
Some of the scenes are priceless.
Or, for a more modern author, try *Barry Fitzhugh. Pest Control, for instance. A struggling bug exterminator is mistaken for a hit man…
I’d like to add a second for Christopher Moore.
I’ve read most of his published books and they’re all hilarious.
Try “Island of the Sequined Love Nun”, “Practical Demon keeping”, or “Fluke”.
Try to keep up. You were actually fifth. Second and third also miscounted, but I’ll give second the benefit of a doubt and chalk that up to a simulpost. 
I’ve also read Barry Fitzhugh and Donald Westlake. I’d suggest them after Moore, Hiassen, Buckley, and Dorsey. I’d even add Brian Wiprud ahead of Fitzhugh and Westlake.
Dave Barry and Patrick McManus are essayists, which isn’t what you asked for. However, Barry does have a novel, Big Trouble, which was big fun.
Of all the stuff mentioned in this thread so far, Tom Sharpe seems to me closest to the sensibility that appealed to the OP in Heller’s Catch-22. Riotous Assembly is utterly appalling and screamingly funny at the same time, and Indecent Exposure isn’t far behind it. The Great Pursuit is probably funnier if you’ve had some connection with the literary world, and ranks behind the others for that reason only. The others are at times almost too bitter and over-the-top, but still hilarious.
Twain is not a bad suggestion, especially in some of the more scabrous and lesser-known works (Letters from the Earth, “Captain Stormfield’s Visit to Heaven”, etc., but ultimately Twain has an affection for humanity that I see no evidence of in Sharpe’s work, and that’s not really a feature of Catch-22 as I recall it, though it’s been many years since I read it.
Carl Hiaassen - by no means great literature, but most of his books have me laughing out loud at least once.
Janet Evonovich’s Stephanie Plum books - also not great literature - in fact these may be classed as complete and total trash, but she always does make me laugh - and it isn’t like they are difficult.
David Sedaris can be funny - I think it sort of depends on how sentimental he feels, but I know a lot of people who always find him funny.
Glory Lane by Alan Dean Foster is in a similar vein.
Hiassen- Go with Sick Puppy.
Just started Fluke after finishing Lamb by Moore, and I agree that he’s hilarious.
Looks like I get to second Tom Robbins. Vonnnegut has some funny stuff, too.
I’ll third the Tom Sharpe. Gut-bustingly funny (and horrible), particularly the two South African ones; and one of my faves, the lesser-known The Throwback. I laughed until I nearly pissed myself reading that one.
Bill Bryson, as someone said, is often laugh-out-loud funny. He’s a travel writer, and some of his stuff is kind of irritating (he always goes for the joke over accuracy), but sometimes he’s fantastic. My mother-in-law gave me a book of travel humor essays once, and late at night, exhausted, I read the Bill Bryson essay about France to my wife. Two or three times in the reading, I had to stop reading because I couldn’t breathe, and I knew the next passage would require tremendous self-control to read; by the end of the essay, we were both in tears from laughing so hard. To this day either of us can say, “That was one sick bird,” and we both lose it.
Daniel
Noone thinks Elmore leonard is funny? Well no not all of them, but how about Get Shorty? Also a couple of out of print detective prodies by Andrew Bergman are very funny, one is called “Hollywood and Levine” and the other title I cannot remeber except that it has a year in the title (1944 maybe) and Humphrey Bogart is the detective.
Also, I haven’t read it in a very long time but “Been down so long it seems like up to me” by Richard Farina struck me as being very funny at the time.
The last thread we had along these lines was this one, although it’s a bit more general than what you’re asking for.
A few more recommendations that I don’t think got mentioned in either thread, but that IMHO qualify as both humorous and literature:
Killer Diller, by Clyde Edgerton, had at least one laugh-out-loud moment for me. It’s a sequel of sorts to Walking Across Egypt, which I also greatly enjoyed but don’t remember as being quite as funny.
Wobegon Boy is possibly Garrison Keillor’s best novel: funny, satiric, and poignant.
Handling Sin, by Michael Malone, is great fun and was a favorite of Birdmonster when I recommended it to him.
And now, since this thread has threatened to broaden out into funny books in general, a recommendation that’s certainly not what the OP asked for, but which I throw out because it’s the funniest book I’ve read recently and because it strikes me as exactly the kind of book Dopers would love. Like a really good Straight Dope column or General Questions thread, it’s well-written, laugh-out-loud funny, and learn-a-lot informative:
Sperm Are from Men, Eggs Are from Women: The Real Reason Men And Women Are Different, by Joe Quirk
Everyone above, plus Gene Weingarten’s ‘I’m with Stupid’ and everything every written by PG Wodehouse. Oh, and I second Christophe Moore.
My problem with The Throwback, funny as it is, is that like a lot of later Sharpe the characters who get the worst of it aren’t necessarily evil enough to deserve it. Yeah, they range from unpleasant to nasty, and I wouldn’t want to hang out with them, but the fates meted out to them seem a bit much. In the South African novels, there was at least the evil of apartheid to serve as a justification for what happens.
Didn’t stop me from naming one of my creatures on the old TechnoSphere online world “Lockhart Flawse”. A nasty one, he was.
You might want to try Christopher Buckley’s books.
Now you’re just messing with me. 
A lot of stuff from Hunter S Thompson is good. I really liked The Rum Diary.
Humorous, factual, historical? You be the judge.
I second the Thompson suggestion (big suprise there, right guys? ).
I suggest The Curse of Lono or Fear & Loathing In Las Vegas.
As for Pterry, he oghtta dedicate the next book to the SDMB, what with us drumming up all the business for him.
Right, Pterry?
I haven’t been able to find Thompson’s novels; I checked the local university library and they are fittingly labeled “Item Missing”. I think the “Good Dr.” would have liked that. I have read a couple of collections of his political ‘essays’ (I suppose thats the word), as well as a Bio about him entitled Fear and Loathing; The Strange and Terrible Saga of Hunter S. Thompson, by Paul Perry. Judging from what I read therein, the second part of your quote pretty much applied to his life as well as his work. The phrase ‘Hunter arrived with a six pack of beer’ (or something similar) appeared so often a non-native English speaker might have mistaken it for his title. And then there is the scene were he spends a job interview trying on hats from a box…which he brought with him…to a job interview…
Most of the other books/authors mentioned are new to me, so I just want to thank you guys again for all the recommendations-I should have enough reading material to faciliate life as a recluse, for the forseeable future anyway.