WikiBooks

WikiBooks is a website that allows for crowd-sourced content (anyone can edit a book). The content tends to be very good, though the look and feel of the website is very utilitarian.

https://en.wikibooks.org/

“Wikibooks is a collaborative book authoring website, where users from all over the world work together to write textbooks and other types of instructional books on many topics. It is a Wikimedia project, operated by the same group of people who run Wikipedia, the Wikimedia Foundation. You can edit this page, and almost all pages like it, at any time. That is the basic principle of Wikibooks: anyone can edit it.”