That line reads as though more than 350 books might actually be detrimental. A slightly more detailed explanation from clicking through the link in the article

That line reads as though more than 350 books might actually be detrimental. A slightly more detailed explanation from clicking through the link in the article: > The study, led by Dr. Joanna Sikora of Australian National University, found the greatest gains in adult literacy, numeracy, and ICT skills when a home had from 80 to 350 books — no additional gains were seen above that number.