https://openstax.org/subjects
They work in large groups, grouped by discipline (topic). It's a voluntary effort. Anyone can join. I've been active on some of the development committees. Whoever "shows up" from CA colleges and universities joins the discussion and provides input.
While the idea of reduced-cost or free textbooks for students sounds noble and appealing, these "collective" writing efforts can have a political bias. That is the obvious danger. There is usually one "free" text per topic, such as Macroeconomics, so it is a monopoly product. No alternative free texts offering different perspectives are provided to students, right now.
Just be aware of this - it is especially concerning in social science subjects, like mine (Economics), as well as for world history and US history, political science, etc. Even physical and earth science "free" texts can harbor political bias (e.g., man-made global warming).
IOW, 'You get what you pay for' still holds... Yes, textbooks are enormously expensive. $100-$200 each and up. But -- there ARE ways an instructor can "diversify" the student's reading list so they become critical thinkers, at a low cost -- for ex., the Mises Institute has free downloads of classic Austrian works students can be assigned, etc.
I'd encourage conservative professors, both adjunct and full-time, at colleges & universities to get involved in these online text collaborations and provide strong input. Or, start your OWN group - and develop an alternative open-source text offering conservative viewpoints & perspectives.
Besides the potential feeding of politically biased books in some subjects, as other posters have mentioned, I wonder about the future of textbooks as more of them move to a free model. In the sciences, textbooks must be updated to incorporate new knowledge and understanding; what is the incentive for authors and publishers if the textbooks are free?