The difference was striking. The difference in later education between having zero books in the home and 500 is the same magnitude as between having parents with a third-grade education and parents with a bachelor's degree. Either having parents with degrees or 500 books pushes a kid further in their education by 2.4 years in the US or 6.6 years in China. All countries combined, it's 3.4 years.
And it scales. Having 20 books makes a difference. A smaller difference than 500, but the more the better, up to 500, where they stopped looking.
This is much cooler than budget meetings. And it's gratifying to know I did something right with my own issue.
UPDATE: I wonder how additive it is. Does having both a BA and 500 books add 4.8 years of education? And how far up does it scale? Does having an MA or PhD or 2000 books have an effect? How much?