“Onions In The Stew” by Betty MacDonald.
A bit dated (takes place during WWII in Washington State), but it’s the true story of a blended family living on Vashon Island. I read it in a Readers’ Digest Condensed Book (oh - the shame!) when I was a kid and just re-read the whole uncut thing - I laughed out loud in a lot of places.
VCNJ~
I’d like to add the travel writing by TIM Moore-- Travels with my Donkey, among others.
Also Jasper Fforde-- strange literary fantasy-ish (very loosely considered) mystery-like novels. Lots of books and lit jokes for the over-educated.
What? No Gerald Durrell? Try My Family and Other Animals
Continuing the famous nature writers who also are knee-slappingly funny, there’s Farley Mowat. He’s most well-known for his Never Cry Wolf, but for his humor, I’d suggest these two: The Dog Who Wouldn’t Be and The Boat Who Wouldn’t Float.
Roald Dahl and Kinky Friedman.
Lucky Jim by Kingsley Amis is the funniest thing ever. It’s a little dated, and kind of sexist, but even after many readings it still has the power to leave me breathless with laughter.
I came in here to mention Fforde - I’m surprised I was beaten to the punch (well, not really, I suppose…this is the Dope). I consider Vonnegut to be very funny. Of his work I’d recommend Cat’s Cradle and, if you like that, Galapagos.
Other stuff if you just want to fill in the cracks on a rainy weekend: James Thurber, Fannie Flagg (particularly Daisy Fay and the Miracle Man), Garrison Keillor, Erma Bombeck, Woody Allen, Bill Cosby and…what the hell? When’s the last time you read Shel Silverstein?
Voltaire’s Candide was quite funny. I seem to recall some of Tom Robbin’s earlier works to be amusing too.
An important note on Prattchett is that he gets a bit absurdist in most of his works, and you said you liked more concrete stuff. If that’s how I’m supposed to read it, I’d recommend you start with his The Night Watch series, which is quite concrete and doesn’t require you to swallow quite as many elephants as the rest of his works does.
So, yeah, give the Night Watch books a go; if you like them, great, you could probably check out his Discworld (Rincewind) series from the start by getting a copy of The Colour Of Magic and finding your way from there. I love him and he’s made me choke with laughter at work, and that’s no mean feat.
Of course, regardless of everything, you should check out Prattchet’s co-op book with Neil Gaiman, Good Omens NOW! Don’t walk, run!
I found that and The Egg and I at a book exchange and started re-reading because I remember loving them as a kid, too. I got bogged down by the casual racism, which passed over my head when I was 8, I guess. Maybe I’ll try one more time.
I am not a huge fiction fan, so if I am to read a novel it pretty much has to be funny. I tend to read the same novels over and over again…some of my favorites:
Anything (of the 6 or so that I have read so far-my favorite being Cat’s Cradle ) by Kurt Vonnegut
Catch 22
Fear and Loathing in Las Vegas
And one I haven’t seen mentioned : The Ginger Man by J.P. Donleavy (mostly due to my love of the absurd and wordplay)
a bit 'o hijack: Hilarity N. Suze, is it even possible to get anything out of Gravity’s Rainbow without the companion? I have had both on my shelf for years but it is kind of a pain in the ass to go back and forth between both books (oh, and the whole thing about it having 1292992921 pages). Maybe I am not smart enough for Thomas Pynchon 
(although I did -mostly- enjoy The Crying of Lot 49)
Catch-22 is one of the few books I have read 3 times. It’s brilliant. It’s also unique. I love humorous fiction, and I can recommend my favorites, but it’s not the same as this, so you may not necessarily like it.
Favorites:
Hitchhikers guide
Are You Dave Gorman?
A Long Way Down
Good Omens
Travelling Ireland With a Fridge
Disagree with:
Princess Bride. Good, but not funny.
Mark Twain. Too much story, too little humor.
David Sedaris. Not funny at all.
Candide. Repetitive. Not funny.
Fiction by Dave Barry. (his non-fiction is really good though.)
That’s the problem I have with the old science fiction classics (which really are worth reading) - very dated, very sexist. I have to consciously suspend my belief that women can be more than secretaries and enjoy them for what they are. It’s interesting from a sociological perspective to read older books like that and see the pervasive attitudes of the time.
It is huge, but there are some very funny bits. I believe my favorite was Tyrone Slothrop’s experience with English “candy” but there were others. But it is huge. I read it by keeping it in the bathroom for a few weeks.
YMMV. I have never been able to get through The Ginger Man.
I’m one of them.
Also:
A Confederacy of Dunces, by John Kennedy Toole
Just about anything by Bailey White
MARK TWAIN!!!
Lords, that man is funny. His words just sparkle on the page. It’s a testament to his literary prowess that his cheekiness feels modern and fresh to me so many years later.