Could someone please explain the ending in "Mostly Harmless" by Douglas Adams?

[Peter_20]

I think this book, for some reason, was very tragic, and the ending feels really dark - but what happened?
I get the feeling that the main characters had given up everything and gone crazy, and then what?
Did the universe itself end?
I remember that Arthur "realized that this was finally the very end", but could someone please explain this?

[edit- lol added spoilers tag, ~rubah]

[Ouch!]

In The Restaurant at the End of the Universe, Author Dent again encounters the creature who he kills over and over again as the creature is reincarnated repeatedly. The creature mentions all the times Author has killed him, and mentions that Author killed him once at Stavromula Beta. Author claims he's never been there, the creature gets upset. Because Author knows that at some point he must kill this creature at Stavromula Beta, he knows he cannot die until he accomplishes this.

The climax of Mostly Harmless happens after a ridiculously twisted plot web of screwed up time lines and other nonsense that had arisen since the fourth book in the series. All the character except for Zaphod end up (again) on Earth before its destruction by the Vogon fleet. They go to a bar called Stavro Mueller Beta before the attack. Author realizes that he had misheard the creature and Stavromula Beta is Stavro Mueller Beta. Vogons blow up earth. Everyone dies. Except Zaphod; he was unaccounted for.

Edit: Adams said he was disappointed by how depressing the book was in his auto-biography-type-thing The Salmon of Doubt. He was sad during his life, and so his book echoed that. He mentioned writing a sixth, but that never happened.