I searched this thread and did not see this site mentioned.
The Connexions site, sponsored by Rice University, generates epubs and pdfs with technical content, sort of like eTextbooks. They cover a range of topics in Math, Science, Engineering, and even Music Theory, and all their content (as far as I know) is DRM-free and freely distributable. I wish I could search their epub/pdf 'collections' from inside Calibre.
http://www.cnx.org