The second edition of "Introduction to Addons" was updated for Celestia Version 1.3.0 (and later). Several new features are available which make it a lot easier to keep track of the Addons you've installed and to move them to the next version of Celestia. Older Addons still work, but if you're willing to change them a little, they'll be much easier to use and understand.
This third edition was updated to include a relatively simple Nebula Addon for NGC3372. It's more complicated than the star and planet Addon but not as demanding as Rassilon's Rosetta Nebula Addon. Also, a section has been included which tries to provide help for common problems.
Some of these example Addons describe imaginary objects. There really is a Rosette Nebula, but we really don't know its three-dimensional shape. Also, we know nothing about whatever planetary systems might be in it.
If you don't tell me that something's missing, unclear or wrong, I can't improve it.