I think THGTTG and 1984 are great, but... I'll offer something new then.
Another by Orwell, This Perfect Day, is a wonderful story, but...
Wizard's First Rule is, I think, is the only book I've read that could be compared to THGTTG. It is in itself a very interesting story, and you can end there, or read the sequels and have even more.
Jonathan Livingston Seagull is short but inspiring. I'd recommend that too... But the one book, besides The Hitchhiker's Guide To The Galaxy, is Wizard's first rule.