One old favorite and one new one:
Mervyn Peake, The Gormenghast Trilogy - Weird surreal fantasy, with a wicked sense of (black) humor. The TV version stank, BTW.
Sarah Maitland, Ancestral truths - One of the few recent novels I’ve read with a non-dysfunctional family that is nonetheless very real.
Ross McDonald- probably the best alround noir writer.
John D. McDonald- the Travis McGee series of books.
Dick Francis- even the old stuff are pure gold.
Dashell Hammett (sp?)- great writer.
-me
For their incredible insight and ubeatable wit:
Tom Robbins
Kurt Vonnegut, Jr
Thomas Pynchon
Three of the best,IMHO.
Another vote for Annie Lamott here. I have four copies of “Operating Instructions” so I can give it to anyone who is expecting.
Others: Ferrol Sams Well, I haven’t tried too hard lately. I have the four books about Porter Osborne, Jr in my permanent library and I reread them every few years. They’re fantastic, awe-inspiring, funny, heartening, and a joy to reread. Once a friend told me she couldn’t get past the first chapter of “Run With the Horsemen” and I thought I was going to burst out bawling.
Elinor Lipman Boy she really hits me just right, her voice and her wit. I like every book of hers I’ve read, even her short stories and I’m not usually a big short story fan.
Kent Haruf I just got turned onto him this year. His spare, clean writing is something else. He’s like these other authors: I read them and I think, “These guys deserve to be at least as rich at Stephen King and Dean Koontz and John Grisham, damnit.”
I give a nod of agreement to Stanislav Lem and Dashell Hammett. Well recommended.
I personally push Rudy Rucker. His latest post-cyberpunk “Transrealist” works are really interesting, check out “Saucer Wisdom” and “Realware.”
Larry Niven, and I’ve been fairly successful so far. I talked a friend who claimed to hate science fiction into reading ‘Destiny’s Road’ and now she’s read several of his works, plus a couple of volumes of ‘The Man-Kzin Wars’, which are anthologies of stories by other authors set in Niven’s ‘Known Space’ future history.
Umberto Eco - In the Name of the Rose and Focault’s Pendulum
Anything by H.P. Lovecraft (the first great horror writer)
Most prose and any poetry by Charles Bukowski
and on the lighter side Dave Berry and Patrick F. McManus
and of course Zecheria Sitchin
William Faulkner
Umberto Eco
Orson Scott Card
Joseph Heller
Harry Crews
Peter Carey
Jim Harrison
Walker Percy
I try to get all my friends to try Mercedes Lackey. I love her world of Valdemar and the heralds.
Go try 'em 
I’ve had more success trying others with Carl Hiaasen than any other author.
I’d like to have had the same success with Paul Watkins - his closest parallel would be Hemingway, but his publishers seem to be keen on repackaging him as a thriller writer. I recently saw a copy of his latest book in the Crime section of a bookshop :(.
Taking of Crime: Edward Bunker. What do you mean, “who”? He was the tough looking Mr Blue in Reservoir Dogs - the guy who never spoke. And he writes staggeringly well.
Ahh…where do I even begin with this?
Julio Cortazar. I haven’t read everything he wrote, but what I have read is great.
Don DiLillo. I’d especially recommend White Noise.
Jorge Teillier, a Chilean poet (post-Neruda).
Someone mentioned Stanislaw Lem in this thread. I loved the Cyberiad, and I’ve recommended it several times to other people.
Somebody else named Rohinton Mistry and specifically mentioned his novel, A Fine Balance. I think Mistry’s a wonderful writer, but I preferred his first book, Such a Long Journey to A Fine Balance, which I found a bit mawkish.
P.G. Wodehouse.
For biological stuff, I’m a fan of Richard Dawkins and Ernst Mayr.
I’ll stop now.
Oops–I spoke too soon. I have just a couple more to throw in…
Robert Bly did a great translations of Neruda’s and Vallejo’s poetry. The translations are different works from the original writing, but Bly is such a good poet himself that you get a whole bunch of incredible interpretations of the original poems.
Lane and…Lane and…Sczeska? (I think I spelled that right.) They’re a team that did several very clever children’s books. I’d recommend them to adults, too. The Stinky Cheese Man and Other Fairly Stupid Tales is terrific, as are The Math Curse and Squids Will Be Squids.
And some more…
I often try to get people to read any of the Best American Short Stories compilations. (A new one comes out every year.) There are a lot of hard-to-find gems of prose in those things.
Ben Hamper. You can either read his essay, “I, Rivethead,” or check out his book, Rivethead. They’re both very angry and absolutely hilarious. If you like black humor and have a sympathy (or empathy) for people who do blue-collar work, you’d love this book.
The Loved One by Evelyn Waugh. That’s a cutting sarcasm, black humor, social satire classic.
Tom Weller’s Science Made Stupid. That should be required reading for all my fellow science geeks.
Ack. Screwed up the coding on that last post.
I read C.S. Lewis’s The Screwtape Letters about a month ago. Check it out!
BTW-I don’t try to push authors down the throat of everyone I meet. But these are all writers I’ve tried to get someone else to read at one time or another.
I read The Screwtape Letters years ago and liked it. Maybe I will read it again.
I also try to turn people on to Ken Follett, specifically his book Pillars of the Earth, which my daughters read and told me about. I read it at their suggestion, and couldn’t put it down until I finished it.