Interesting read about work on the sequel to the game they never got past early development. I think Spellbreaker was the hardest infocom game I played besides this. Hollywood Hijinks was probably the most fun one I finished. Or maybe is was fun because I did finish it.
http://waxy.org/2008/04/milliways_infocoms_unreleased_sequel_to_hitchhikers_guide_to_the_galax/
http://www.infocom-if.org/games/games.html