I often find myself in the situation of having to recommend books and introductory material on Haskell to students. On the Haskell website https://www.haskell.org/documentation/ there is a list of beginner books. A quick check shows that among these books only three, “Real World Haskell”, “Learn you a Haskell” and “Developing Web Applications with Haskell and Yesod” are freely available. (In a HTML version, and not in the typesetted pdf form).
While we have a well-stocked university library which our students can use, I still prefer to recommend material that is freely available on the internet. Maybe the Haskell foundation could try to contact the authors and publishers to see whether it is possible to make some of these books available as PDFs.
Some of these books have been available for several years, and these books are usually not particularily profitable for the publishers or authors anymore.
Several of the books have been published by Cambridge University Press, a non-profit publisher which is open to allowing freely available PDF versions of their books. (Cf. the preface to Tom Leinster’s book on Basic Category Theory: https://arxiv.org/pdf/1612.09375.pdf).
There is also precedence for companies sponsoring the open-access availability of books published by for-profit publishers, and several other programming languages have books that are simultaneously available in print and for free online.
Many thanks again to everyone responsible for setting up the Haskell foundation and making the joy of programming in Haskell available to more people 