Some of the world's best and most sophisticated software is distributed today under "free" or "open source" licenses, which allow the recipients of such software to use, modify, and share it without paying royalties or asking for permissions. If this works for software, could it also work for educational resources, such as books? The economics of software are different from the economics of book publishing, but careful attention to how the free software movement has developed--especially its focus on quality rather than cot--could facilitate a similar development in the domain of educational resources such as textbooks. Those interested in exploring the possibilities of free educational resources should pay careful attention to the concepts of modifiability and modularity, which have been major selling points of open source software. When applied to educational resources, these concepts may call into question the "textbook" as a natural unit. (Contains 5 endnotes.)