I think the sixth HHGG "book" people are thinking of is the short story "Young Zaphod Plays It Safe", included in a lovely leatherbound omnibus edition of the first four books.
I'd just like to add a wonderful bit of description from "So Long and Thanks For All the Fish" (which I rather liked, to be honest). Describing the Californian mystic/seer/prophet/whatever Wonko the Sane: "He looked like someone had taken David Bowie and attached additional David Bowies to the arms and legs of the first David Bowie and then wrapped the lot in a dirty beach robe. He was tall and he gangled." (That's from memory, so forgive the inaccuracy)
Adams had the wonderful twin gifts of a thoroughly twisted imagination and a knack for clearly and eloquently describing the products of that imagination. We've lost a master of the language, friends. Let's all raise a glass for him when we get to Milliway's.
