An open-source project consists of more than just the source code. There is also associated documentation. Some of this is internal documentation used only by the project's developers that can be covered by the same licensing as the code it describes. Other documentation, however, such as user guides, tutorials, and reference manuals that are intended for widespread distribution, have different licensing needs than the source code, especially if they will be published as books.
The point of using an open-source model for documentation is both to make it freely available to as many people as possible and to make it as good and up-to-date as possible by having many people write and review it. Both the freedom to distribute a document and the freedom to modify it are essential. Note that there are many documents that can be freely downloaded for personal use but that are protected by copyright and cannot be modified except by the original authors.
Documentation is different from source code in many important ways. Although both can be downloaded for online use, people often want the convenience of a printed version of documentation, and a commercially published book is usually of better quality than a copy you print for yourself. There are major costs involved in writing and publishing a book, so potential authors and publishers have concerns about how many people will actually buy the book--as opposed to reading it online or printing their own copy--and also about other publishers reprinting the book and selling it. Any documentation license needs to address these concerns. Note that whereas code is usually developed to solve a problem for its writer ("scratch your own itch"), books are written mainly to earn money for the author and to solve the readers' problems.
There is also the question of the author's reputation. When you read a book, you directly experience the author's words. This is unlike software, where a user never sees the source code, only the effect of running it. Often a book has a single author, and if there are problems because the text has errors or is difficult to read, then the reader will rightly blame the author. Likewise, the reader will give the author credit for clear writing and good information. If anyone can modify the text and distribute a new version, then the author might be blamed for errors someone else introduced, and the author's reputation could suffer. So making sure that any third-party modifications are clearly marked as such may be important to the author.
Documentation licenses need to specify whether commercial redistribution is permitted, what types of modifications are allowed and how they are to be identified, whether translations are allowed, whether the source text must be made available (and, if so, whether it must be in a nonproprietary format), and how to credit the original author and publisher. Like source licenses, documentation licenses usually specify that there is no warranty and that aggregation with other documents doesn't cause the license to apply to them.
Some licenses mention good practices for showing common courtesy to authors, such as contacting the authors well before redistributing any large number of copies, to give them a chance to provide an updated version of the document, and offering them a free copy of any published book or CD-ROM.
One commonly used license is the Open Publication License, a cooperative effort of O'Reilly Publishing, New Riders Publishing, the Open Source Initiative, the OpenContent Project, VA Research, and others, written in 1999. The license was designed to be friendly to commercial publishers, such as O'Reilly and New Riders, who have published several books using the license--the full text of many of these books is available on the publisher's website.
The license is fairly short. It allows for commercial redistribution and modifications, provided that any modifications are identified and dated, that the original author and publisher are acknowledged, that the location of the original unmodified document be identified, and that the original authors' names not be used to endorse the new work without permission.
The license has two options that can be specified to prohibit the distribution of substantively modified versions without the explicit permission of the author and to prohibit any publication in book form for commercial purposes unless prior permission has been obtained from the copyright holder.
Note that an earlier, related license, the Open Content License, is often casually referred to as the "OPL." So if you see a reference to the OPL you will need to check to see which license it really indicates.
A copy of the Open Publication License can be found on the OpenContent website.1
The GNU Free Documentation License (FDL) is similar to the Open Publication License (with neither option specified), but it specifies everything in much greater detail. The FDL was written in early 2000. It is used by many open-source projects where the source code uses the GPL, such as the GNOME project. A number of books using the FDL have been published commercially.
The FDL distinguishes between the main technical content of a document and secondary sections that focus on the publisher or author, such as a preface or acknowledgments. These secondary sections can be declared invariant and required to be included in any derived works. Each author can also specify material that must be included on the front or back covers of any derived works.
People making modifications must add a history section to document their changes. A machine-readable copy of the document, in an accessible (that is, nonproprietary) format, must be made freely available.
A copy of the GNU FDL can be found on the Free Software Foundation website.2
The Sun Public Documentation License (PDL) is a revision of the Mozilla Public License modified to apply to documentation. It was first used in August 2002 as the license for OpenOffice documentation. It was designed to facilitate revisions to existing documents, requiring that all modifications be documented, but not going into the detail that the FDL requires. It also makes it possible to take pieces from one document and use them in another, provided that reasonable attribution is provided.
A copy of the PDL can be found on the OpenOffice website.3
A copy of the FreeBSD Documentation License can be found on the FreeBSD website.4
The Creative Commons5 is an organization "devoted to expanding the range of creative work available for others to build upon and share." It provides a set of licenses aimed primarily at artists, musicians, photographers, and writers that permits a variety of rights to consumers of the work, such as the right to create, distribute, and perform derivative works. These licenses can be used for documentation and, perhaps with some modifications made in conjunction with the Creative Commons, for software source code.
The Creative Commons in many ways exemplifies the principles and core values of the open-source community. Its website is a good place to visit to get a feel for how an even balance between the rights of the author and the rights of others can optimize opportunities for innovation.