I would absolutely read the rest of the Hitchhiker's series and maybe even So Long and Thanks For All The Fish as well (sometimes that gets included, sometimes not).
What else I'd strongly recommend (speaking of little books that carry a big stick), "Illusions: Adventures of a Reluctant Messiah" by Richard Bach (short read but one of my all time favorites. I re-read it ever few years for a recharge.
Also:
"Anthem" by Ayn Rand.. if you want a HUGE book that is in my top 3 books of all time.. give Atlas Shrugged a look-see! Very, very good.
"The Alchemist" (author's name is escaping me right this second) Another short, fast, fantastic read.
For a longer and more "fantasy" based read.. one of the all time best reads EVER has to be the Dragonlance Chronicles (books 1-3). Dragons of Autumn Twilight, Dragons of Winter Night and Dragons of Spring Dawning. If you've never read them.. DO IT.
Oh, if anyone cares to check it out, I've written a series of air-combat articles for an online flight simulation website inspired by the IL2-Stormovik series including "Forgotten Battles" and "Aces Expansion Pack". It's only been up for about a week and already getting close to 2000 hits.. which suprised the hell out of me. You can check it out at:
http://www.il2flying.com
Click on the "Knowledge of Aces" section and I'm in there. Weeee!
Late,
~O Rugs.