I don't have a better source, but Freakonomics had a whole chapter dedicated to the fact that having books impacts learning outcomes much less than having money

I don't have a better source, but Freakonomics had a whole chapter dedicated to the fact that having books impacts learning outcomes much less than having money. While giving kids books and encouraging reading at home seemed like a logical solution, it seems like after school programs, summer camps, etc. are a better predictor for success