The Guide is more than just a 5 book trilogy. It is a creative concept. Adams did a lot of versions--Books, radio, TV, and he did things bit different each time.
That probably seems trivial, but I think it interesting because (excluding the American film) Adams was usually the author. It's like his goal wasn't purity of presentation, but "what would be funny right here."
I'm sure you've all got your towels, and know that if you ever find yourself thrown at the ground, it's best to try to miss, so I'll leave you with a bit that's a tad more obscure, but equally important IMO: Don't look to deeply into anyone else's eyes. You never know how far from home they are. (Related; avoid an accurate sense of perspective at all costs.)