The definition is rather simple: they are textbooks and online classes licensed under open copyright licenses (e.g., Creative Commons). Since they are freely available, you can either read them online or download them as PDFs at no cost. (You may, however, have to pay a small amount if you wish to get a hard copy.) So this list is essentially a free online textbook library. Here, we’ve gathered hundreds of links to open textbooks and online courses from different websites and sorted them by subject.