Celestia User's Guide
by Frank Gregorio

English MS Word document (for Celestia 1.6.0, zipped)

German PDF file (for Celestia 1.6.0-1)
 German Handbook incl. scripting guides by Ulrich "Adirondack" Dickmann

French MS word document (for Celestia 1.6.0, zipped)
 French translation of 1.6.0 guide by Jogad

Dutch MS word document or PDF file (for Celestia 1.6.0)
 Dutch translation of 1.6.0 guide by Marco Klunder

Russian MS word document or PDF file or Online (for Celestia 1.6.0-1)
 Russian translation of 1.6.0 guide by Leserg

Chinese MS word document (for Celestia 1.6.0-1, zipped)
 Simplyfied Chinese by Hanghang

Italian MS word & OO ODT document (for Celestia 1.6.1, zipped)
 Italian by Carlo Pignatti

Romanian MS word document (for Celestia 1.3.2, zipped)
 Romanian translation by MindToAsk

There's no official FAQ for Celestia yet. However, Selden Ball maintains a list of frequently asked questions here on the Celestia forum.
Celestia Wikibook
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.
Additional Documentation
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.

Celestia is Copyright (C) 2001-2016, Celestia Development Team