Check out http://www.douglasadams.com/creations/infocom.php they have the original game online.

The trilogy was actually a trilogy for a while and was so established as a trilogy that he continued to refer to it as a trilogy even when the fourth and fifth books were written. The were one or two short stories added in the later omnibus editions.

My favorite thing about the books was Adams' brilliant was of stating the profound, "The large yellow ship hung in the sky in exactly the same way that bricks don't."