> - there is research supporting the point that "how many books are in their household is a positive predictor" not only of future reading ability but also a bunch of other things, but there seems to be no research supporting that "Growing Up Surrounded by Books Could Have Powerful, Lasting Effect on the Mind".

This is self-contradictory. Research that supports “X is a predictor of Y” which does not inherently refute “X is a cause of Y”, in and of itself, is research supporting “X could be a cause of Y”.