Depending on how the site is configured at any given time, different users can add book pages, such as this help document. The ability to create book pages is normally assigned to "roles" in the administration area.
It's very easy to add a page. When you feel like writing a piece of documentation about a topic that hasn't already been addressed, just click on "Create content" link in the block under your user name. Then click the "Book page" link. Type away in the Body. Depending on administration settings at a web site, you may have various formatting options. Here on drupal.org, by default you will have "filtered HTML" capability.
Give your page a short, descriptive Title.
You will have to set the correct parent to page, select it from the Parent drop-down menu.
Optionally you can add a log message to explain why you wrote the documentation.
Give your page a weight. Pages with a less heavy weight will stay at the top within the parent section while pages with heavy weights will sink deeper.
Then preview and finally submit. Pages may fall into a moderation queue and won't be available for viewing until a moderator approves the page.
