Via LogBlog, (le permalien du billet cité ici ne semble pas fonctionner), le début d'une liste de manuels de logique librement disponibles, établis par Rob Loftis :

I've decided that my students should not have to pay for a logic textbook. Most textbooks are obscenely expensive., but logic textbooks in particular get in my craw. The formal systems that they teach have been a part of the human intellectual heritage for over a hundred years, and the textbooks don't do anything in particular to make more approachable for students. They survive on the laziness of instructors, not on any originality content or presentation.

Open Access Logic Textbooks