My high school ingrained it into our heads that a book isn't truly read unless the reader 'interacts' with the book. We were taught to write down questions that came up throughout the reading, to summarize (in a few sentences) the chapter once we got to the end, to write down our thoughts/observations in the margins when they came up, etc etc etc. I can't help but do this now when I read books. It makes it hard to loan any books to anyone (since it's written so heavily in), but I feel that writing in a book is like joining in on a conversation, where just reading it and not taking any notes is simply listening in on a conversation from a distance.