Textbook

https://en.wikipedia.org/wiki/Textbook

Free
http://www.freetextbooklist.com/

Hyper
https://hypertextbook.com/

Mantra
http://www.textbookmantra.com/

Centre
https://textbookcentre.com/

Tube
https://www.youtube.com/results?search_query=textbook

Slides
https://www.slideshare.net/search/slideshow?searchfrom=header&q=textbook