Most of my open-minded friends read a lot. Most of my non-open-minded friends don't. It makes sense that the more you read, the more likely you are to be challenged by new ideas and that your commitment to your own ideas will weaken. Reading also frees you from everyday life and shows you the variety and richness of life. Therefore, reading makes people open-minded.
 If you selectively read certain kinds of books or books with certain opinions, you are likely to become paranoid the more you read most strongly undermines the conclusion above.