Well, if you haven't yet read The Hitchhiker's Guide to the Galaxy, do it =)

Then some other titles which come to mind easily. I'll try to pick one book from each author;

Stanislaw Lem : The Cyberiad
Joseph Heller : Catch-22
Arkadi & Boris Strugatski: Stalker
Didier Comès: La Belette (I don't know how it is translated in English. This one is a graphic novel)
(At first I thought I won't mention Hugo Pratt, because his works are so well known, but maybe I do:
Hugo Pratt, pretty much anything by Hugo Pratt, yes =) )
Petteri Hannila : Fargoer (I admit I haven't yet read it myself, but this one is partly inspired by UrW)

And finally, if you feel bold, try an all-time classic of Finnish literature: Seven Brothers by Aleksis Kivi