[yongjik]

Yes, the Foundation series is short and great, but only the original. I think the later additions were a let down.

The Dispossessed, by Ursula Le Guin, if you like to explore different social systems and its implications. Great, but might not be everyone's taste.

The Hitchhiker's Guide To The Galaxy, obviously, which is half dry British humor and half nerd humor. I probably missed half of the British humor part, but even so it's really great. Recommended if you want to spend a weekend laughing.