Forall X – An Introduction to Formal Logic

It's an open source textbook so professors will tweak it to suit the course they specifically want to teach. There may be parts of it that they aren't fans of or would prefer to be done a different way so they can make those changes they want and share the ebook with their class.

Ideally more classes would be able to do stuff like this but given the fairly "fixed"/closed source nature of most textbooks, this isn't really an option for most professors or courses.

The subject matter covered is so ancient & timeless that there are plenty of cheap "dover reprint" type books, and even books in the public domain. The spirit of open access / open source in relation to textbooks is commendable of course, but I don't think $100 is the benchmark here.

Point taken (and agreed), but it doesn't really. A boolean logic isn't going to get you sequential circuits, for example. You need recursion for that.