You learn what you read and remember, not what you buy.
It would be a good thing to buy books if one could also buy the time to read them; but one usually confuses the purchase of books with the acquisition of their contents. To desire that a man should retain everything he has ever read, is the same as wishing him to retain in his stomach all that he has ever eaten. He has been bodily nourished on what he has eaten, and mentally on what he has read, and through them become what he is. As the body assimilates what is homogeneous to it, so will a man retain what interests him; in other words, what coincides with his system of thought or suits his ends. Every one has aims, but very few have anything approaching a system of thought. This is why such people do not take an objective interest in anything, and why they learn nothing from what they read: they remember nothing about it.
- Arthur Schopenhauer