The whole "Hitchhiker's Guide to the Galaxy" series - british humour at it's best.
Jerome K. Jerome's "Three Men in a Boat" - the hilarious misadventures of three men travelling down the Thames in a row boat. There's also a sequel of sorts, "Three Men on the Bummel".
"1066 And All That" - a spoof English history written by a public school don, very funny if you are familiar with the sort of English history taught in British schools.
I also second the P.G. Wodehouse mention - wonderful stuff.
And Roald Dahl...the childrens books are classic but he's also done a whole load of adult short stories which are brilliant and very dark.