The "The Hitchhiker's Guide to the Galaxy" is a sort of hand-held encyclopedia which includes an entry for every planet of the galaxy, or as least every one that has been reviewed. The Guide puts out new editions periodically. Ford Prefect first ended up on Earth as a reviewer for the guide. (He was stranded here for a number of years.) In one alternate universe, his entire entry on our world was reduce by editorial intervention to the words "Mostly Harmless".

The Hitchhiker's Guide to the Galaxy is a series of books about the Hitchhiker's Guide, by the insanely funny Douglas Adams. This five book long trilogy (as it is known) was first produced as a radio show, and part of it was later made into a movie. Here are the titles of the books in the series:

The Hitchhiker's Guide to the Galaxy
The Restaurant at the End of the Universe
Life, the Universe, and Everything
So Long and Thanks for all the Fish
Mostly Harmless

Again, I cannot emphasize enough how DAMN funny these books are. I want to reread them just thinking about them...