- Free logic textbook http://www.fecundity.com/codex/forallx.pdf 45 comments philosophy
- Free set theory textbook from the Open Logic Project. http://builds.openlogicproject.org/courses/set-theory/ 10 comments math
- Modal Logic for Open Minds, Bentham. Free modal logic textbook pdf http://fenrong.net/teaching/mljvb.pdf 4 comments philosophy