The writer Umberro Eco belongs to that small class of scholars who are encyclopedic, insightful, and nondull. He is the owner of a large personal library (containing thirty thousand books), and separates visitors into two categories: those who react with “Wow! Signore professore dottore Eco, what a library you have. How many of these books have you read?” and the others—a very small minority—who get the point is that a private library is not an ego-boosting appendages but a research tool. The library should contain as much of what you do not know as your financial means … allow you to put there. You will accumulate more knowledge and more books as you grow older, and the growing number of unread books on the shelves will look at your menacingly. Indeed, the more you know, the larger the rows of unread books. Let us call this collection of unread books an antilibrary.

When you buy a book, you establish a property right in it, just as you do in clothes or furniture when you buy and pay for them. But the act of purchase is actually only the prelude to possession in the case of a book. Full ownership of a book only comes when you have made it a part of yourself, and the best way to make yourself a part of it— which comes to the same thing— is by writing in it.

Why is marking a book indispensable to reading it? First, it keeps you awake— not merely conscious, but wide awake. Second, reading, if it is active, is thinking, and thinking tends to express itself in words, spoken or written. The person who says he knows what he thinks but cannot express it usually does not know what he thinks. Third, writing your reactions down helps you to remember the thoughts of the author.

Reading a book should be a conversation between you and the author. Presumably he knows more about the subject than you do; if not, you probably should not be bothering with his book. But understanding is a two-way operation; the learner has to question himself and question the teacher. He even has to be willing to argue with the teacher, once he understands what the teacher is saying. Marking a book is literally an expression of your differences or your agreements with the author. It is the highest respect you can pay him.