> In one of the Hitchiker books (I don't remember which one, but I would guess that it was "Life, the Universe, and Everything"),

Actually, it was the very first one, _The Hitchhiker's Guide to the Galaxy_.

> the main idea was that, after spending eons calculating the great Answer to Life, the Universe, and Everything, the greatest computer
> said that it was "42." But what was the question?

One of the theories I quite like is the one based on a scene in _The Restaurant at the End of the Universe_, suggesting that the Question is
"How are we for time?"