I must admit that I thoroughly enjoy "So long, and thanks for all the fish" (book number 4). Of the five books, it is the one I have read the most times. I'll agree that "Mostly Harmless" (number 5) fell far short of its predecessors though.

Yes the are brilliant, but why not listen to the "original" radio series where it all started. Easily found on iTunes.

I've heard them. Once was enough. I'm glad I did though, but I prefer the books.

If you enjoy Douglas Adams, and obviously you do since you have the intelligence to locate a keyboard and type on a forum, there is one book in particular that you just can not miss.
Last Chance To See.
Wherein he travels the world with a zoologist looking for endangered species. It is the absolutely best book I have ever read, even knocking that off Hitchhikers and Lord of the Rings off their respective perches.

I've not read it but I know that it was the companion book of a BBC Radio series, which is getting a television remake featuring Mark Carwardine (who wrote it along with Douglas Adams) and Stephen Fry. It begins in September in the UK on BBC Two and has a website here.
I'm not sure about your ability to see it in Norway, but I imagine it will likely end up with a DVD release if it popular.

Great books. After seeing that Richard Dawkins and Douglas Adams were such good friends, my girlfriend decided to start reading the books.

He did. She worked on Dr. Who.

Personally I've always thought HHGTTG should be taught in sociology and anthropology courses..

I love the movie just because Stephen Fry voicing the Guide... :P

Fair point, but Adams took his degree in philosophy, and I think his books are better suited to that than any discipline besides literature. More so with the Dirk Gently ones IMO, but it's there in HHG too. He really has some thought experiments that are flat out brilliant (the electric monk, for instance).
Why Adams is not read more in literature courses boggles the mind. Great, great writer. After Wodehouse, probably the premier English comedic novelist. Personally, one of my favorite 20th century writers after Chesterton and Borges. His use of personification for humorous effect borders on godlike.
Who also listened to the BBC radio series? It's available on CD now. How are the later volumes based on the last two books? I haven't heard them.

So, now you finally know why the hell "42" shows up everywhere. ;-)
I don't remember how young I was when I first read these, but I do know that they were great then, and they're still great now.

Not at all. The books are based on the radio series, but takes the story in a different direction. In fact, every time HHGTTG has changed formats, the story has been changed as well. As far as I recall, the last few radio shows go off in a completely different direction than the books do.

What about Terry Pratchett? Better than adams could ever hope to be

Actually, I think Pratchett's books get kinda repetitive and predictable after you've red a few of them. Don't get me wrong, I LOVE Pratchett, I've bought a few books by him just recently, however - well, he can't beat guide to the galaxy and last chance to see. Those books are just too brilliant. Of course, it kinda comes down to personal preference in the end :D