I doubt very many parents read the textbooks, at all.
________________
I do.
I also read every textbook in sight before putting it in front of my homeschooler.
By the way, a lot of the material we use isn’t “textbook.” For example, in history, there’s an old collection called Annals of America. Chronologically organized volumes of American writing, so you grasp history as viewed by the people witnessing it at the time.
And one of my favorite educational resources is youtube.com, though we have at home a very extensive collection of educational films. Youtube’s got more! :)
My biggest complaint about recent textbooks is that they are presented as values-neutral, and they’re often merely anti-American; the contrast with older books is remarkable.