The Unofficial Guidebook to the Galaxy and Beyond was a book that attempted to provide a rough outline of every inhabited planet in the galaxy. Due to the immenseness of this task, the publishers invited anyone with knowledge of any planet to contribute to the book via the HoloNet. This greatly increased the amount of entries, but also resulted in a lot of misinformation and factual errors. Nonetheless, the Guidebook became very popular, and was in use during the times of the Empire and beyond.
Behind the scenes
The Guidebook is based on both Wikipedia and the Hitchhiker's Guide to the Galaxy.