Although the book itself hasn’t directly been given an award, author, Douglas Adams has. In 1984, Adams became the youngest person to receive the Golden Pan award, an award that is given to an author whose books sell over one million copies. The entirety of the series has sold over 15 million copies, with Mostly Harmless fitting into that.
The movie, however, did receive a direct award. It was nominated for seven different awards, and had won one of them. That being, the Golden Trailer Award under Most Original. If anyone has either seen the movie or read the books, this award makes complete sense.
Mostly Harmless 