Using GNU FDL
If you know someone who is writing a manual about free software, and looking towards commercial publication, you have a chance to help the Free Software Movement a great deal with a small amount of work: by suggesting the idea of publishing the manual under the GNU Free Documentation License.
Until recently, commercial book publication almost always implied a nonfree book. But just as free commercial software has been increasing for a while, now free commercial documentation is starting to take off as well. Some of the major commercial publishers of documentation about the GNU/Linux system, and about free software in general, are now willing to publish books under the GNU FDL, and pay the authors in the usual way—IF the authors are firm about this.
But publishers are likely to first propose an ordinary proprietary book. And if the authors agree, that's what it will be. So it is essential for authors to take the lead; to say, “We want to use the GNU FDL for this book”. So when your friend mentions writing a manual, you can influence the course of events simply by pointing out this possibility.
If a publisher rejects the request at first, the GNU Project may be able to help the authors prevail. They can contact us at <email@example.com>.
There are other legitimate free documentation licenses, but sometimes using them requires care. For example, one license is equipped with two optional clauses that can be enabled; the license is free if neither optional clause is used, but enabling either of them makes the book nonfree. (See http://www.gnu.org/licenses/license-list.html.) Authors that want to publish free documentation, but using a license other than the GNU FDL, can contact us so we can check that the license really qualifies for free documentation.
When a commercial manual is free and relates to the GNU system, the GNU Project can recommend it to the public. So if the authors or publisher set up a web page to describe it and/or sell copies, we can make a link to that page from www.gnu.org/doc/other-free-books.html, provided the page meets our usual criteria (for instance, it should not link to other pages about nonfree software or documentation, and should not be obnoxious about trying to persuade people to buy). Please inform <firstname.lastname@example.org> about such pages.
See also Free Software and Free Manuals.