The documentation here is copious, but still incomplete. Webmin is a huge project with over a hundred standard modules and a couple hundred third party modules. Jamie's book covered most of the standard modules that existed in 2003 in reference form, and Joe covered a popular subset with more of a focus on teaching concepts. In both cases, the documentation has become somewhat dated and Webmin has advanced far beyond where it was back then. Modules are easier to write, themes easier to build, and capabilities in some of the core modules have become quite advanced. Cluster features have become central to the Webmin way, and lots of new modules have been introduced or modified to support multiple servers. So, many folks in the Webmin community have discussed the documentation problem ... and the obvious solution is a wiki!
Now we need your help. Here's what you can do:
Don't be shy
If you find find a mistake or omission: don't hesitate but hit the edit tab above and start editing. Should you be making mistakes yourself, they can (and hopefully will) be corrected soon enough.
The sources for most of the documentation are old, often terse, and now incomplete. So add to it and clean it up. When you find that something isn't covered that you think is interesting, edit the page and talk about it. Even if you aren't an "expert" on the subject, tell folks what worked, and stick a "Warning" in to indicate you aren't sure how correct your advice is. All of us can probably pool our knowledge and make something useful out of it. If you find something that isn't right, fix it. If you find something that is grammatically stupid, fix it.
Also worth noting, the documentation was imported from a half dozen different formats and the conversion was incomplete to varying degrees. Fixing this stuff up will be an ongoing process.
Screenshots and Videos
In general our screenshots didn't make the transition from the various original sources, and pictures are worth 1,000 words. So, we need to get new screenshots. There are hundreds of images referenced within these pages, and most are no longer accurate. You can upload them directly into the wiki, or upload to Flickr or imgur and link to them (and if you tag them "webmin docs" or "usermin docs" we'll like you a lot...we have some plans for making using images from Flickr and imgur easier).
And in a world with YouTube, there's no reason not to make video tutorials of common tasks. Grab Wink or Gnome Shell Screen Recording and make some movies of your favorite Webmin tricks, and upload them to YouTube and add a link.
We don't cover much third party modules, and some of them really rock. If you've written a third party module, and want to talk about it, create a page for it, and add it to the Modules page in the Third Party Modules by category section. Even if you didn't write the module, if you use it and understand it, tell us about it!