In the case of a textbook, this begs the question:  “Why are you providing your book to everyone at no cost?”  I can’t answer either question for your particular situation, but I can give you my reasons for using an open license for my textbook, A First Course in Linear Algebra.

A promise.  By giving readers and teachers an unrevocable right to make and distribute copies, there is no edition-churn and no possibility of the book going out-of-print.  It is a promise that the book will always be available.  If you have ever built a course around a book, just to see it go out-of-print, you’ll understand.  And because I also distribute the LaTeX source, anyone else can maintain the text if I become unable, or uninterested, in continuing to do so.

Quality control.  I believe readers are more inclined to contribute corrections when they know you are providing the book for free.  My experience with my book and Judson’s abstract algebra text is that the quality can far surpass that of commercial texts.  And by making corrections available rapidly via Twitter, and by posting new editions, these corrections can reach readers very quickly.  I now offer my own students $5 for each mathematically significant error, confident that they will not find very many.

A responsibility.  Notice too, that if I am unreasonable in accepting legitimate changes from contributors, anyone is free to fork the book and distribute the new version containing their changes.  So I have placed myself under some obligation to be reasonable and prompt about responding to suggestions and making corrections (and you know that prior to using the book).

Improvements for all.  If you make changes to my book (hopefully improvements), and distribute the changed version, then you must use the same license.  In this way, I require that changes by others remain available to all.

Duplication.  Why do I have over thirty introductory linear algebra textbooks in my office?  Do we need that many?  Certainly, my book will not work for every course (for example, it would not be appropriate for a class full of engineering students) but maybe three or four quality open texts on linear algebra would be sufficient.  I would rather contribute one of these few texts and work to see it become as effective as possible.  When I started in 2004 I would say, “The world does not need another linear algebra text, but it does need a free linear algebra textbook.”

Another reason for not using a commercial publisher: I have never missed a deadline on my book project.

So this should help explain why I have chosen the GNU Free Documentation License (GFDL).  I have some strong feelings about licenses that include a non-commercial clause.  More on that later.

I am frequently asked about licenses for open textbooks.  I will try to only insert a modicum of opinion here and save my views for later, both the positive and the negative.

It is important to realize that an open license works with copyright.  In the United States, every creative work automatically has a copyright, even if it is not stated (e.g. “Copyright 2011″) and even if there is no registration with the government.  This page you are reading now is copyrighted.  A copyright is a monopoly, granted by government, to allow you to control the distribution of your creative work.  In the United States, this monopoly runs for your entire life, plus another 70 years for your estate.

With an open license, you give others greater rights to your work than copyright allows.  At a minimum, an open license allows others the right to make copies, and to distribute copies, forever, and you may not revoke that right.

There are two principal licenses used for textbooks.  One is the GNU Free Documentation License (GFDL).  It will feel familiar to users of open source software, since GNU is the organization that supported much of the free software that surrounds the Linux kernel.  The other is the Creative Commons license, which comes in a variety of flavors, depending on the combination of options used.  This license is often used for a variety of media, such as music and photographs, so it is not as careful about talking about source material for a work, such as the LaTeX source for a mathematics textbook.

Here is a list of the common licenses, with a brief informal description of their features.

• CC-BY: Unrestricted copying, distribution and reuse in other works of any kind.  Your only obligation is to ensure that the author is always acknowledged (“attribution”) as the creator of the work.
• CC-BY-NC:  Adds the requirement that you may not use the work in a commercial way.
• CC-BY-ND:  Stipulates that you may not produce a derivative work.  In other words, the work must remain intact.  For example, you could not produce a screenplay from a novel, or except a poem from one collection to add into another.
• CC-BY-SA:  You may do everything allowed by a CC-BY license, but now you are obligated to license any derivative work with a CC-BY license.  This feature is why this license is called a “viral” license.
• GFDL:  This is another viral license, and is very similar to a CC-BY-SA in its main features.  However, with its roots in licensing documentation for software, it is more explicit about making source material available as well.  It also explicitly mentions textbooks as one possible use.
• CC-BY-NC-SA:  This combination should be obvious.
• CC-BY-NC-ND:  The most restrictive.  You can make and distribute copies for noncommercial purposes and cannot reuse or extend the work.

How does these licenses work in practice?  What are the advantages and disadvantages of the various options?  What are my opinions about each?  (Hint: I use the GFDL for my linear algebra book.)  I’ll have lots more to say in subsequent posts and likely refer back here often.