I've read the whole trilogy of five books (yes, but that is how Douglas Adams worded it) twice over. The first book had some really funny stuff in it that was left out of the movie.
BUT... Adams wrote 2 versions for BBC radio, a BBC TV series, a BBC movie, and the books, and finally the screenplay. Every single version was slightly altered. So if you like the book and what it's about, you can't say the movie was bad since it was his screenplay. Yes, it might have left out some parts, overstated others, etc but it was the version he tried for 10 years to put on the big screen. He died without ever knowing that a movie would be reality.