The X New Developers Guide (2013)

https://www.x.org/wiki/guide/

Low Level X Window Programming: An Introduction by Examples (2017)

https://link.springer.com/book/10.1007/978-3-319-74250-2

Freely available X Windows System manuals from O'Reilly:

https://x.org/wiki/UserDocumentation/

To make it clear, no one is buying X.org

seems a bit of a bait post given recent announcements. But this kind of thing has always annoyed me, where companies overload a name, especially a super simple name. Makes it hard to search for, especially searching on the non-dominant name of something.