Oooh, good one! Also, The Snake Oil Wars, the sequel, is quite good. I think there’s a third, but I’ve never read it. Both of those are in my “comfort, humor, reread” collection.
url=http://www.amazon.com/gp/product/0671638106/sr=8-1/qid=1153975598/ref=pd_bbs_1/002-9161653-5447225?ie=UTF8]The Book of the SubGenius will give you a larf and save your soul! 
One of the best writers of satire I’ve ever run across is Jean Shepherd. The movie A Christmas Story is based on his stories (and he narrarated it, too). You might try Wanda Hickey’s Night of Golden Memories, and Other Disasters, or In God We Trust, All Others Pay Cash
Other than that, I’ll second David Sedaris and Dave Barry.
Bill Bryson is incredibly funny- the sort of funny that leaves you laughing hysterically on public transport as you read, with the other passengers making nervous glances towards the exits and wondering if they can quietly slip off the train at the next stop.
P.J. O’Rourke has done some great stuff, too (I thoroughly recommend Holidays In Hell), and Harry Harrison’s Stainless Steel Rat series doesn’t really qualify as “Funny”, but it is thorougly tongue in cheek and not intended to be taken entirely seriously.
Golfing For Cats by Alan Coren is quite a good read, and Scott Adams (of Dilbert fame) has written a number of semi-serious Dilbert books that are very amusing, tongue in cheek, and worth checking out- I especially recommend The Dilbert Principle, The Dilbert Future, and Dilbert and The Way Of The Weasel.
Some of the more notable stand-up comedians have written surprisingly good books- Drew Carey’s Dirty Jokes and Beer and Jeff Foxworthy’s No Shirt, No Shoes, No Problem are surprisingly good and well worth a look.
It goes without saying that the Red Dwarf books are must-reads, and Rob Grant’s book Colony is very much in the same style.
Not all of Ben Elton’s books are humorous, but many of them are, and if you like Red Dwarf and Blackadder, then you’ll probably enjoy his work as well.
Not really novels, but the “Jetlag Travel Guides” to fictional countries- currently Molvanvia: A Land Untouched By Modern Dentistry and Phaic Tan: Sunstroke On A Shoestring- are quite funny and will no doubt raise a smile or two.
Have a look in Remainder Bookstores for relatively obscure books by one or two-off authors- I’ve found quite a few great little humorous books with titles like Doomsday: The Survivor’s Guide, How To Be An Evil Dictator, and The Action Hero’s Handbook in remainder bookstores and the like. 
I will second PJ O’Rourke. I especially enjoy Holidays in Hell. Great, great stuff.
Bill Bryson makes me want to walk the Applachian Trail even with Katz** and all over England. ( I haven’t made to his other books as of yet, but he has a wonderful way of observing things in a semi-romantic yet cynical way.
Gil’s All Fright Diner is horror lite with heavy doses of laughter.
This coming from a woman who will not watch scary movies or read horror novels. (I do like paranormal stuff.) I’ve been in the forbidden world of Romance Novels for two decades [size=1]I’m not proud. and the only scary movie I’ve ever seen has been The Thing, TYVM. Back when it came out 20+ years ago. I am a total pussy and that’s ok.
The title of the book alone jumped out at me off the New Release section at the library. I read the back cover and was intrigued. The Author’s thank you part cracked me up and after the first page, I was hooked.
I’ll shut up now.
I’m sure if I re-read it it would probably collapse like a house of cards, so grab it/read it before my enthusiams wanes.
In Regards to the Phoenix Guards, it looks to be a part of a series. Is it necessary to read the entire series or what?
As it seems Terry Pratchett has been mentioned…over and over. I’ll be more specific and mention Good Omens. Terry Pratchett AND Neil Gaimen. Channeling Douglas Adames. You need this book.
I didn’t say i could spell
I found The Sex Lives of Cannibals by J. Maarten Troost outrageously funny. It put me in mind of Last Chance to See, which really is a must-read for any Douglas Adams fan.