Thanks for being āopenā enough, Martin, to ask a fundamental question. The answer is much more than one book or resource, we are talking about an entire field of practice, research that has been ongoing for decades.
As @Munirmoosa suggests, its more a way of thinking, believing, though I might not use only ācostā as a barrier that we seek to remove.
There is a fairly well-written definition in what may be one of the hallmark examples of the potential of public knowledge creation, Wikipedia (not without some issues, thatās another discussion).
Itās baked into many things, including the name and the statement from organization I work for
We envision a world where everyone, everywhere has access to the high-quality education and training they desire; where education is seen as an essential, shared, and collaborative social good.
and you will find more from the ānodeā or subgroup of Open Education Global that is focused on open education at community college, CCCOER, or more broadly vocational and adult education sectors in other parts of the world.
But we do not own any definition or profer to have the ābestā.
You of course will come across hundreds, thousands, or definitions out there. Maybe more importantly is the one we craft for ourselves, and not only write, but is seen in our actions (I am an idealist).
But yes, it is much broader than course materials, and what is used in academic halls. Keep asking these questions, and for those reading out there, please add thoughts, ideas here to guide Martin (and others).