I think its also heavily regionalized.

So California tends to set the standard for the Pacific Coast / Mountain West. Texas book suppliers are heavily influential in Gulf Coast and Mississippi value school system. Florida and Virginia heavily impact the Atlantic Coast. New York and Massachusetts weight the NE corridor.

I also know there's a not-insignificant amount of education that is Texas-specific. There's a state requirement for middle schools to teach Texas History, as distinct from US or World History, for instance. And Texas science textbooks got caught up in the big fight over teaching evolution and sex ed, back in the 90s. So there's a real bright line between what a biology, chemistry, and human health book will teach in the mid-00s, entirely dependent on which side of the Rockies you went to school.