Adler - How to Read a Book
Only books of that sort will make you stretch your mind. And unless you stretch, you will not learn.
Thus, it becomes of crucial importance for you not only to be able to read well but also to be able to identify those books that make the kinds of demands on you that improvement in reading ability requires. A book that can do no more than amuse or entertain you may be a pleasant diversion for an idle hour, but you must not expect to get anything but amusement from it. We are not against amusement in its own right, but we do want to stress that improvement in reading skill does not accompany it. The same goes for a book that merely informs you of facts that you did not know without adding to your understanding of those facts. Reading for information does not stretch your mind any more than reading for amusement. It may seem as though it does, but that is merely because your mind is fuller of facts than it was before you read the book. However, your mind is essentially in the same condition that it was before. There has been a quantitative change, but no improvement in your skill.