AbeBooks asked its UK customers what books make them laugh, and based on those responses compiled this list:
Top 10 Funniest Books According to AbeBooks.co.uk Customers
1. Right Ho, Jeeves by P.G. Wodehouse (1933)
2. Catch-22 by Joseph Heller (1961)
3. The Hitchhiker’s Guide to the Galaxy by Douglas Adams (1979)
4. Three Men in a Boat by Jerome K. Jerome (1889)
5. Wilt by Tom Sharpe (1976)
6. A Confederacy of Dunces by John Kennedy Toole (1980)
7. Lucky Jim by Kingsley Amis
8. The Code of the Woosters by P.G. Wodehouse (1938)
9. Bridget Jones’s Diary by Helen Fielding (1996)
10. Adolf Hitler: My Part in His Downfall by Spike Milligan (1971)
From this site, which also contains some analysis of the list — not to mention comments from said customers.
From the list, I’ve read only Hitchhiker’s Guide in its entirety; I’ve read part of Catch-22 (and intend to finish someday). I’ve had Wodehouse on my to-read list for quite awhile.