I think a lot Clifford Simak’s books might be good choices.
I came in to recommend the Fuzzy Trilogy. The first books is Little Fuzzy. They’re by H. Beam Piper.
As Cal said, Brown and Sheckley are good bets.
Clarke’s “Tales From The White Hart”.
Asimov did write some humorous stories. He also wrote a number of shaggy dog short-short stories (ending in horrible puns).
I did find “Laughing Space”, a 520-page (!) anthology edited by Asimov. I’ve never seen it, and I’ve know only a couple of the stories from elsewhere, so YMMV. It’s not in print, so you’d have to get a used copy. http://www.amazon.com/LAUGHING-SPACE-Isaac-Asimov/dp/0395305195
You can get 50s radio productions of sci-fi stories here X Minus One - Single Episodes : Old Time Radio Researchers Group : Free Download, Borrow, and Streaming : Internet Archive. Most are dramatic, but I can verify “Early Model” (by Sheckley) is funny. “The Coffin Cure” is here and it’s also in the Asimov anthology. (The text is on Project Gutenberg, too, which is a bit surprising, because it seems too recent to be out of copyright.)
Yet another person recommending Connie Willis’ To Say Nothing of the Dog.
It’s not just that it’s funny and lighthearted, it’s sweethearted and romantic without being cloying. It’s uplifting. Maybe even life-affirming. It deals well with the idea of time travel and the idea of paradox. The characters, even the disagreeable ones, are sympathetic. And my favorite part, other than the romance?
The story is set about 2050, I think, and at that time, domesticated cats are extinct and have been so for at least a generation. The laws of time travel dictate that a time traveler cannot bring anything back from the past or take anything from the present to the past. By the end of the story, the Oxford professor of time travel discovers that you can bring forward something which has become irrelevant to the time stream - like, oh, kittens thrown in a pond to drown. The two main characters are given two to raise at the end of the story.
I always finish that story with a silly, happy grin on my face.
What about John Scalzi’s Old Man’s War? It’s pretty entertaining and reasonably intelligent, if it’s not great literature. And I don’t recall any moment in which it was all that depressing.
Also, there’s Kage Baker’s novels of the Company, starting with In the Garden of Iden. They’re a little depressing in places, but mostly they’re satires.
Daniel
I love To Say Nothing of the Dog, but when I read it I regretted throughout that I hadn’t read her earlier time travel novels that kind of “set out the rules” more completely. (And there is no way in hell I can send him The Doomsday Book. Obviously.) I mean, not everybody’s like me - I always feel robbed if I read things “out of order”, even if there isn’t a real order.
I don’t know; I had no hesitation in giving To Say Nothing of the Dog to my mother who had read none of the previous stories. The only important information for To Say Nothing of the Dog is that Willis’s time travel is done as university projects through Oxford and that it shifts the destination a bit if the traveler would create a paradox. It’s true that knowing the reoccurring characters gives it a bit more depth, but the story vital exposition is all there.
By the way, the next book in the Oxford time travel series should be out next year - Connie reported at Balticon in May 2008 that she had finished the initial draft, so I’ve been eagerly awaiting “All Clear” since then.
I’ve never read this author. Can anyone recommend where to start?
She’s most famous for her short stories and novellas, as Just Some Guy pointed out. The Winds of Marble Arch is a great new collection of her best stories, but it’s an expensive hardback. There are other collections you can get in paperback.
Doomsday Book comes first in her time travel series, but it is a very serious book (about the Black Plague) where To Say Nothing of the Dog is a comedy. I think they both work well as a standalone novel. Her other novels are all standalone - *Bellwether *is one of my favorites. I did not like the couple of books she wrote with Cynthia Felice.
I’m really glad to hear she has a new novel coming out soon.
Depends on what you’re looking for. Her short stories are awesome, but her novels are fantastic, too - the thing is, they’re all very different. I’d suggest either To Say Nothing of the Dog (Victorian time-travel, very funny, riffs on Three Men in a Boat) or The Doomsday Book (medieval time travel, heartbreaking and amazing). I’ve read To Say Nothing of the Dog characterized as the first Hugo winner that was funny on purpose, if you know what I mean. Or just pick up a collection of her short stories; I believe “The Last of the Winnebagoes” is a famous one.
Of course, a lot of the characters and situations in The Stupidest Angel are even better if you’ve read his previous novels. The Stupidest Angel is probably the closest thing to a sequel Christopher Moore had until he came out with an actual sequel to one of his earlier books last year.
Maybe Lamb: The Gospel According to Biff, Christ’s Childhood Pal would be good for a depressed religious person. It’s lighthearted, and I don’t think it would be considered blasphemous. And it has a relatively happy ending!
Well, unfortunately today I got the word that you can send him packages, but not books. (Somebody probably send A Canticle for Leibowitz last year and made somebody hang themselves with their shoelaces.) I’m going to pick up a few and give them to him when he gets back, though.
Send him a Kindle and tell them that it’s a calculator. 
I’d vote for John Varley. He has a wonderful swashbuckling space-opera tone to his books, though with hard-scifi themes underlying them, that make them wonderful reads.
How can people feel better if they’re cut off from reading (that is, if they’re readers)? That makes me very sad.
I’ve read the Demon Princes series by Jack Vance more times than I can count.
Well, there are times I’m up for the latest depressing holocaust prison camp black slavery massacre of Native Americans flick, and times I really just want happy. I don’t see why it would be different for reading.
I was responding to the news that books couldn’t be sent to the gentleman.
Oh, okay. Sorry.