You forgot a book. Mostly Harmless came out in 92.I guess I have to spell it out for those who have not read the "Hitchhiker's Guide to the Galaxy" 4 book run. (Hitchhiker's Guide to the Galaxy. Life, the Universe and Everything. The Restaurant at the End of the Universe. So Long and Thanks for all the Fish (with a special literary cameo by David Bowie.))
Long story short, the quest, which had included the construction of Earth as a supercomputer run by mice, only to be destroyed to make way for a hyperspace byway, was to find the answer to the question of life, the universe, and everything.
Finally, a computer was built that could answer questions. One scientist asked, "Is there a God?"
The computer answered, "There is one now,"
Anyway, finally, the computer came up with the answer to the question of Life, the Universe, and Everything.
And that answer is 42.
Some had considered that maybe the computer was inadvertantly working on the math problem of 6 x 7, but no, it could not possibly be that.
There is also a 6th. Eoin Colfer was given permission by Douglas adams wife to write And Another Thing in 2009 for the 30th anniversary of the first book.