books may be one of the few places where children who are socially isolated and insulated from the larger world may meet people unlike themselves. If they see only reflections of themselves, they will grow up with an exaggerated sense of their own importance and value in the world-a dangerous ethnocentrism.
What this clause is saying is that reading literature is like everything else. They have their pros and cons. The cons here is that if children read books and if they act more like mirrors, they will start to think that they are dominant in terms of strength and intelligence as others. In other words, over-encouragement.