If you were a medieval scholar reading a book, you knew that there was a reasonable likelihood you'd never see that particular text again, and so a high premium was placed on remembering what you read. You couldn't just pull a book off the shelf to consult it for a quote or an idea.
Moonwalking with Einstein: The Art and Science of Remembering Everything (ed. Penguin, 2011) - ISBN: 9781101475973