The natural first answer for me is Wodehouse, a base that has already been covered. Just the general use of language in his novels puts me in a wonderful mood, even if the plots as such aren't necessarily overly funny.
I've just read Jerome K. Jerome's "Three men in a boat" yesterday, and will highly recommend that as well. It's essentially a boating-trip story, chock-full of digressions, mixing both comedy of a slapstick nature with plenty of hilarious sentences of similar nature to Wodehouses.
The most famous quote would be: "I like work: it fascinates me. I can sit and look at it for hours."
The whole book's public domain and available online too, if you'd like to sample it, or even read the whole thing:
Three Men In A Boat
Kingsley Amis's Lucky Jim is a bit less overt than those two, but the novel has a wonderful way of feeling like it's constantly on the verge of spinning completely out of control. The main protagonist isn't making life easy for himself, and at every corner another plate drops off his pole and risks utter catastrophe. This constant tension and the results of his conduct makes for a ton of fun.
For something that isn't really "hilarious", but which manages to be be both a beautiful novel, and a very funny one at the same time, try Evelyn Waugh's A Handful Of Dust. It's a fine satire, so now that I think back on it, I mostly remember less humorous aspects of it, yet I also recall chuckling heartily many a time whilst perusing its pages.
Interestingly, it seems all my favorite humor-texts are British. Vonnegut is the only American author that comes to mind, but I'd highly recommend him as well (and, like most of the aforementioned authors, there's thankfully more his work than strictly comedic writing)