Okay, kiddo, let me tell you about books of authority. Let's start with what "authority" means. Authority is like a person who knows a lot and you can trust them to teach you something correctly. Like your teacher, they know how to solve math problems and you can trust them to teach you the right way to do it.
Now, books of authority are books that people trust to teach them about something. For example, there is a book called "The Elements of Style." It's a book that lots of people who write trust because it teaches them how to write well.
Same goes for textbooks at school. They are books of authority on different topics because we trust that the information they contain is correct and helpful for us to learn.
Some books of authority are really old, like religious texts such as the Bible or Quran. People have trusted these books for a very long time to teach them about their beliefs and how to lead their lives.
Overall, books of authority are simply books that people trust to give them accurate information on a particular subject or topic. They are like a shortcut to learning because if you read a book of authority, you already know that the information is trustworthy.