The Free Software Movement has had notable successes that have inspired efforts in other fields. After all, if people can collaboratively create an operating system or a video editor then share it over the internet, could they not do the same for, say, a textbook? And, in fact, a few people have done just that.
Most recently, a Kickstarter funding campaign to professionally design a university textbook in Philosophy achieved its desired result. Clear and Present Thinking can be downloaded as a free pdf file or read on the Internet. A Kindle version is available. There's also an inexpensive version in solid format, suitable for reading in the bath. Read the details on the author's web page. Congratulations to the author, Brendan Myers, for his achievement, and good luck with the next steps: a French translation and an expanded version of the text.
Speaking of French, there's a university-level introductory textbook in French called Liberté by Gretchen Angelo. It comes in both student and instructor editions. Instructors can also get MP3 files to accompany the text.
Liberté is hosted on lightandmatter.com, a web site that was created to house free university Physics textbooks by Ben Crowell. Light and Matter itself is meant for a one-year introductory course "of the type typically taken by biology majors,
or for AP Physics B." Simple Nature is for those who are more comfortable with calculus, so is meant for "engineering and physical science majors,
or for AP Physics C." There are also Mechanics, Conceptual Physics, General Relativity, and Calculus. The PDF's are free downloads, and inexpensive printed copies are available.
Dr. Crowell's been busy.
I'm sure his Calculus book is as readable as his other ones, but there are many on-line recommendations of Sylvanus P. Thompson's Calculus Made Easy, especially the first edition, as the best, or at least the most approachable, calculus textbook ever written. As it was first published in 1910, it has shown remarkable staying power. Project Gutenberg has a freshly typeset edition of the 1914 edition.
For many years, the University of Toronto published a textbook called Representative Poetry. That book is now on-line and extended thanks to its editor, Ian Lancashire. Now at version 6.0, it is "a web anthology of 4,800 poems in English and French by over 700 poets spanning 1400 years."
Community College Consortium for Open Educational Resources tries to categorize and link many free and open textbooks, but beware. They are not all at the same level of development and quality. For example, it links to an Art History on-line textbook, Smart History, which has been nominated for an internet award, but it also links to a Paleoanthropology "textbook" that is more an outline than a finished product.
Another "everything but the kitchen sink" site is Freebookspot.es. I could find a useful variety of PDFs on the site. However, an ad blocker is probably warranted.
So, we can see some progress in the provision of free textbooks for universities and colleges, especially for the first year of studies, but what is there for high schools and high schoolers? Since inexpensive textbooks are most important in poorer countries, I would expect worthwhile books to come out of, for example, Africa. In fact, a physicist named Mark Horner created a project called Free High School Science Texts (FHSST). The books are now on the Siyavula web site, are called Everything Maths and Everything Science, and are available for grades 10-12 in English and Afrikaans. The same site has Natural Sciences and Technology workbooks for grades 4 to 6.
For texts to read in English class, as well as in other languages, Project Gutenberg and its Canadian and Australian equivalents are worth a visit. Why buy a copy of Macbeth or "The Devil and Daniel Webster" or 1984 if reading it on your computer or tablet for free is acceptable? All legally, in Canada, I hasten to add.
Finally, just because, I want to mention Basic French for Canadian Schools, a textbook that was published by the Government of Ontario in 1937. It is out of print, but the pages were scanned and collected into a PDF file. It would be wonderful to have someone turn it into text and do the necessary proofreading to make a nice, new edition. Unfortunately, I'm going to be a bit too busy to take on that task for quite a while!