I remember when I was in high school in Baltimore County (Maryland) we used text books from a Yale University project for some advanced math courses -- Algebra III & IV and Analytic Geometry. They were as close to a cooperative, open source text book you could get in the 1960s. They were also excellent.
California has the largest public school population in the US and along with Texas, New York, and Florida command the majority of the attention of the textbook publishers. Many other states just follow what these states adopt for textbooks.
Open source will also be a boon for the at home schooled children as they will now have access to approved textbooks without the high cost.
Coupled with the easy availability of the classic literature through PG (and our own download section), this is a great step forward.