Just another GR refugee. Other than that, I had a stroke in 2004, and read almost anything I can get my hands on, though I have a particular weakness for history, mystery, and historical fiction.
All of these titles, in different moods and at different ages, have made me laugh in public.
When I was a teenager, The Hitchhiker's Guide to the Galaxy was the book all the other teens were reading, and it was hilarious. My parents gave me My Life and Hard Times at that age, and it was a wonderful, very funny memoir of growing up in the midwest a century ago, from one of the founders of The New Yorker (not to be missed: "the dog that bit people" and "the night the bed fell").
Encountered as an older adolescent, and again in college anthropology, Motel of the Mysteries had me laughing non-stop (my roommates, too, as I recall). The illustrations are half the fun. At about this time I also read The Code of the Woosters, a masterpiece by P.G. Wodehouse.
More recently I've read, loved, and laughed at Poetry for Cats and Lucky Jim. (The latter I was reading while visiting my father; he heard me howling with laughter from upstairs and then nodded with understanding when my answer to "What's so funny?" was "Lucky Jim." He gave it to me, by the way.)