Adams, Douglas: So Long, and Thanks for all the Fish

The Hitch Hikers' Guide to the Galaxy 4.
Just when Arthur Dent's sense of reality is at its most clouded, he suddenly finds the girl of his dreams. He finds her in the last place in the Universe in which he would expect to find anything at all, but which 3,976,000,000 people will find oddly familiar. They go in search of God's Final Message to His Creation and, in a dramatic break with tradition, actually find it. PART FOUR IN A TRILOGY OF FIVE.
Arthur Dent is out of his bathrobe, in love, and wondering why the dolphins said...So Long and Thanks for All the Fish. Was the earth really demolished? Why did all the dolphins disappear? What is God's final message to His creatures? Arthur Dent, Ford Prefect, and the new voivoid gang are off (by commercial airline) on a wacked-out quest to answer these truly unimportant questions.END