Well, sometimes books express an author's view on something. For example, if it's a book about the future, that could be the author's opinion on what they think the future will be like. Or if you are reading a book about human rights, that probably reflects how the author feels about the subject. So, by reading these books on certain topics, you see how the author feels about the topics, how the author thinks about the topic, etc. It's kind of like having a conversation with friends. For example, if you were talking about the future, and a friend said that they think it's going to have flying cars and everyone will live forever, and they explain it, that is very similar to how some authors write books.