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.