I read the Hitchhiker's Guide to the Galaxy in elementary school and loved it. Later in high school I re-read it as well as three (of the five) other books in the "trilogy". I still need to read Mostly Harmless. If you liked the first book I would recommend reading the whole collection.

I enjoyed the movie for what it's worth. There is obviously no way the movie could incorporate every aspect of the book but I did find it fairly entertaining and in the same spirit as the books. I think Douglas Adams would be proud.