(If you want DVDs of the scans of the individual pieces of paper
forms on which weather data have been
traditionally recorded, you'll have to pay production costs, but they are
available.)
The deal is this: a course instructor selects online the text that he or she wishes to use for a course; because it's in digital
form, the instructor can, and is free to because of a Creative Commons license, modify the text in any way desirable; once firmed up, the text is made
available online to students, who may use it online for free but who will pay in order to be able to download the book for printing or for use offline; the cost to the student is perhaps 20 % of what a
traditionally published text would sell for; authors obtain royalties.