The funniest writers did not write novels. Maybe you ought to try anthologies of short humor by America's greatest humor writers. These include Robert Benchley, James Thurber, Dorothy Parker, and Stephen Leacock (who was actually a Canadian). James Thurber's A Thurber Carnival would be a good book to start with, and still easy to find.

E. B. Write (author of the children's books Charlotte's Web and Stuart Little) was a humorist. He edited a big book titled A Subtreasury of American Humor, which you ought to be able to find at a good library. It introduces you to all the American humorists.

It suddenly occurs to me that you might like the stories and novels of P. G. Wodehouse. His stories about Jeeves are the best known.

I agree that David Sedaris is laugh out loud funny, but he writes autobiographical essays, not fiction.  And I have to disagree that Dave Eggars' Staggering Genius is funny.  It is dry and has moments of dry humor, but the overall tone is actually a little depressing and I found the book terribly slow.

A few "free time" reads that made me laugh include:

• The Fault in Our Stars by John Green (also very sad, but good)
• The Pleasure of My Company by Steve Martin (short and silly)
• Bridget Jones' Diary by Helen Fielding
• The Princess Bride by William Goldman
• Forrest Gump by Winston Groom

I also will agree with Kurt Vonnegut - especially Cat's Cradle or the short story "Harrison Bergeron" which is entertaining. If you are looking for something less dense, I laughed out loud at the young adult novel A Fault in Our Stars although I also sobbed. True range of emotions!

Adventures of Tom Sawyer and Adventures of Huckleberry Finn are also good calls as marbar57 noted earlier. They are poignant but comical and ironic.

I, too, agree with the above posters. I adored The Perks of Being a Wallflower. Outside of that, I would suggest David Eggers' A Heartbreaking Work of Staggering Genius and Christopher Moore's The Stupidest Angel: A Heartwarming Tale of Christmas Terror.

I am a big fan of David Sedaris and his writings on his observations of family and life in general. He is laugh out loud funny at times because you can completely imagine the situation he is depicting. One of his most famous is Me Talk Pretty One Day. 

I would also recommend The Absoluletly True Diary of a Part Time Indian by Sherman Alexie, and The Perks of Being a Wallflower by Stephen Chbosky.

I agree that Kurt Vonnegut, Mark Twain, and Joseph Heller are sure to make you laugh. Currently, I'm reading Infinite Jest by David Foster Wallace. It's very sarcastic and funny on many levels. It's a very heavy book, though, so beware.

Depending on your taste and whether you like very dark humor, you might want to check out Chuck Palahniuk, author of Fight Club and some other fantastic books. 

The website below, Goodreads.com, is a great place to find suggestions for books to read.

http://www.goodreads.com/list/show/5447.Funniest_Novels_of_All_Time?auto_login_attempted=true&format=html&page=2

If you like really absurd humor, the Douglas Adams series Life, the Universe, and Everything is pretty funny.

Joseph Heller and Tom Robbins could usually make me laugh until I hurt. Especially Heller's Catch 22 and Robbins' Even Cowgirls Get the Blues.

I laugh out loud at few novels, some of which are included in the previous two responses. But the one that never fails to make me laugh is A Confederacy of Dunces by John Kennedy Toole. Like most good comedy, it is bittersweet, and has a strong element of tragedy to it, but Toole creates characters and situations that I find hysterical. Ignatius J. Reilly is for me one of the most original, funny characters in American literature. 

I guess I'm pretty old-fashioned but I really like the novels of Mark Twain.  I recently read Tom Sawyer (for the 3rd time) and am currently reading Huckleberry Finn.  I got the biggest kick out of both of them! They are both funny because they are both a satire, which means they poke fun at people and their practices.  The language of the locals is particularly humorous. Try them both . . . and enjoy!

Two more of Mark Twain's novels I still intend to read are Life On The Mississippi and The Jumping Frog of Calaveras County.  I hear they're both pretty good, too. 

The novels of Kurt Vonnegut are always funny - bitter and cynical, but funny. I'd also suggest a book called The Askby Sam Lipsyte. Not to build the book up too much, but it made me laugh out loud more than a few times. If it strikes your sense of humor this book could be worth your time.