Fair point. It is free in HTML format on its page. And as the authors said (on the page you linked) "We make the content freely available online. If you like it

Fair point. It is free in HTML format on its page. And as the authors said (on the page you linked) "We make the content freely available online. If you like it, please buy a copy." This looks like a trend: to provide a free on-line copy and still sell "better" formats.