Celestia Users Guide
Celestia Key Chart
by The Learning Technologies Project Office of NASA
CEL Scripting Guide
SSC File Scripting Guide
by Ulrich "Adirondack" Dickmann and Bob Hegwood
PDF document (zipped)
STC File Scripting Guide
Virtual Textures for Dummies Package
There's no official FAQ for Celestia yet. However, Guckytos maintains a list of frequently asked questions here
on the Celestia forum.
The Celestia WikiBook
contains technical documentation on Celestia
for users, add-on creators, and developers. Experienced Celestia users
are invited contribute to the Wikibook themselves.
Please see the documentation page
at The Celestia Motherlode for a comprehensive list of documentation available for Celestia. In addition to documentation for Celestia users, there are many files of interest to people who want extend Celestia by creating add-ons and scripts.