If I had to name only one source, it would be M. Helitzer - Comedy Writing Secrets - the best book on comedy writing I've ever read. It explains why certain things are funny, describes techniques for brainstorming and writing jokes, and it's also a pretty funny book.
My favourite humorists are P.G. Wodehouse (Jeeves or Castle Blandings novels and short stories), David Lodge (The British Museum Is Falling Down, Changing Places) and Woody Allen (short stories). Personally I consider T. Wolfe a very funny author, especially his latest novel I Am Charlotte Simmons.
I'd also recommend listening to some stand-up comedy: George Carlin (he's the best), Eddie Murphy, David Cross, Bill Hicks, Dane Cook...