[userbinator]

I can understand calling these "texts", but "books"? These remind me more of the sectioned documentation pages that one often comes across on the "document web"; e.g. https://tldp.org/HOWTO/Plug-and-Play-HOWTO.html

To me, "book" implies constant-sized pages and content formatted such that it could be used to produce an actual hardcopy book (e.g. a PDF).

[anamexis]

"Textbook" often has a connotation of being written to the needs of teaching a particular course, which I think "text" does not.

[pfortuny]

Lecture notes is what I would call these (as I do my materials).