This quote means that what you see in books is based on what you already know and believe. In other words, books show you what you already have inside you. For example, if you already have a good understanding of a concept, then when you read about it in a book, you will be able to pick out and understand the details that relate to what you already know. On the other hand, if you don’t know much about a topic, then the book won’t make sense to you because you won’t be able to spot the details that are related to the topic.