I'd recommend some of the books by Robert Sheckley, Mindswap particularly, if you like a somewhat surreal sci-fi based humour (translation: his writing may well not be to everyones taste!).
And, of course, Douglas Adams to add to Terry Pratchett.
I would also add the Isabel Dalhousie Sunday Philosophers Club books by McCall Smith; its not that they're funny, it just I grew up in Edinburgh and they are some cuttingly observed that I can't help laughing. From time to time.