You did recommend those, but maybe you just meant the first one. All five are hilarious.

Have you read any of Douglas Adams' other stuff? The Dirk Gently series? I've only read The Long Dark Tea Time Of The Soul. It's a great book but nowhere near as funny as the Hitchhiker series.

If you like Douglas Adams then you'll love Catch 22 by Joseph Heller, if you haven't read it already. It's slightly harder going than the Hitchhiker series, but written much more eloquently.. and it's alot more confusing. And - dare I say it? - alot funnier at times.