It's not quite a math book, but Richard Zach in the Dept of Philosophy at the University of Calgary wrote to let me know of the Open Logic Project. The project is a collection of open educational resources (OER) aimed at teaching advanced logic courses typically found in philosophy curricula.
I looked over the web site and they appear to be adhering to the spirit of OER. The materials, including a pair of logic textbooks, have source available and are hosted on GitHub.
Of course, one of the major challenges of OER adoption is simply awareness. Please share this with your colleagues in the philosophy department.