Ask HN: Who are the big names in Open Textbooks?

I'm looking to build something cool to accompany open education content.

(a) Who are the big names in open textbooks? (b) Are there textbooks where the source is open to contributions and not just the final product? (c) How does the CC license work with the Practice problems at the end of open textbook chapters?

Open Textbooks [1] https://cnx.org/ [2] https://open.umn.edu/opentextbooks/ Mixed Textbooks [3] https://ocw.mit.edu/courses/online-textbooks/ [4] http://composingprograms.com/