This quote is encouraging people to read and think critically about books. It means that when you read a book, you should think about it more deeply than just a casual glance. Some books can be read quickly and enjoyed without much thought. Other books should be read slowly and carefully to really appreciate the content and digest the ideas. Finally, there are a few special books which should be read slowly and thoughtfully, and these should be thoroughly digested in order to better understand and appreciate the ideas in them.