The Hitchhiker's Guide to the Galaxy, Douglas Adams, (1979)

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 The characters visit the legendary planet Magrathea, home to the now-collapsed planet-building industry, and meet Slartibartfast, a planetary coastline designer who was responsible for the fjords of Norway. Through archival recordings, he relates the story of a race of hyper-intelligent pan-dimensional beings who built a computer named Deep Thought to calculate the Answer to the Ultimate Question of Life, the Universe, and Everything. When the answer was revealed to be 42, Deep Thought explained that the answer was incomprehensible because the beings didn't know what they were asking. It went on to predict that another computer, more powerful than itself would be made and designed by it to calculate the question for the answer. (Later on, referencing this, Adams would create the 42 Puzzle, a puzzle which could be approached in multiple ways, all yielding the answer 42.)

The computer, often mistaken for a planet (because of its size and use of biological components), was the Earth, and was destroyed by Vogons to make way for a hyperspatial express route, five minutes before the conclusion of its 10-million-year program. Two of a race of hyper-intelligent pan-dimensional beings who commissioned the Earth in the first place, disguise themselves as Trillian's mice, and want to dissect Arthur's brain to help reconstruct the question, since he was part of the Earth's matrix moments before it was destroyed, and so he is likely to have part of the question buried in his brain. Trillian is also human but had left Earth six months previously with Zaphod Beeblebrox, President of the Galaxy.

babel fish “Now it is such a bizarrely improbable coincidence that anything so mind-bogglingly useful could have evolved purely by chance that some thinkers have chosen to see it as the final and clinching proof of the non-existence of God.
The argument goes something like this: "I refuse to prove that I exist,'" says God, "for proof denies faith, and without faith I am nothing."
"But," says Man, "The Babel fish is a dead giveaway, isn't it? It could not have evolved by chance. It proves you exist, and so therefore, by your own arguments, you don't. QED*."
"Oh dear," says God, "I hadn't thought of that," and promptly vanishes in a puff of logic.
"Oh, that was easy," says Man, and for an encore goes on to prove that black is white and gets himself killed on the next zebra crossing.”

*quod erat demonstrandum (that was demonstrated)

You may not go Back from here not even with ruby slippers, sorry.