Since I love both teaching and open source development, I suppose it was only a matter of time before I attempted a survey of open source text books. Here are my notes on the projects I've come across so far:
Light and Matter
The Light and Matter series is a set of six texts by Benjamin Crowell at Fullerton College in California. The series is aimed at the High School and Biology (i.e. low calc) audience. The source is distributed in LaTeX and versioned in Git. I love this guy!
Crowell also runs a book review site The Assayer, which reviews free books.
Radically Modern Introductory Physics
Calculus Based Physics
Calculus Based Physics, by Jeffrey W. Schnick at St. Anselm in New Hampshire. It is under the Creative Commons Attribution-ShareAlike 3.0 License, and the sources are free to alter. However, there is no official version control, and the sources are in MS Word format :(. On the other hand, I wholeheartedly agree with all the objectives Schnick lists in his motivational note.
Calculus Based Physics' Schnick linked to Textbook Revolution, which immediately gave off good tech vibes with an IRC node (#textbookrevolution). The site is basically a wiki with a browsable list of pointers to open textbooks. The list isn't huge, but it does prominently display copyright information, which makes it easier to separate the wheat from the chaff.
College Open Textbooks
MERLOT's Open Textbook Initiative
The Multimedia Educational Resource for Learning and Online Teaching (MERLOT) is a California-based project that assembles educational resources. They have a large collection of open textbooks in a variety of fields. The Light and Matter series is well represented. Unfortunately, many of the texts seem to be "free as in beer" not "free as in freedom".
Open Access Textbooks
The Open Access Textbooks project is run by a number of Florida-based groups and funded by the U.S. Department of Education. However, I have grave doubts about any open source project that opens their project discussion with
Numerous issues that impact open textbook implementation (such as creating sustainable review processes and institutional reward structures) have yet to be resolved. The ability to financially sustain a large scale open textbook effort is also in question.
There are zounds of academics with enough knowledge and invested interest in developing an open source textbook. The resources (computers and personal websites) are generally already provided by academic institutions. Just pick a framework (LaTeX, HTML, ...), put the whole thing in Git, and start hacking. The community will take it from there.
Anyhow, everything I've read about this project smells like a bunch of bureaucrats churning out sound bytes.
Finally, there are a number of textbooks on arXiv. For example, Siegel's Introduction to string field theory and Fields are posted source and all. The source will probably be good quality, but the licensing information may be unclear.