Laura Miller has an interesting piece in Salon about the joy of writing in books:
Marginalia is a blow struck against the idea that reading is a one-way process, that readers simply open their minds and the great, unmediated thoughts of the author pour in. In reality, reading is always a collaboration between reader and author, and even the basic act of underlining a passage represents a moment in the individual, unrepeatable experience that one person had with one book on one particular day. The underlining itself reminds us of that. You might return to the book later and wonder what you possibly could have seen in the underscored sentence, why you singled out that one instead of a much better one on the next page. Or you might even relive an epiphany from your youth.
This is one of those topics that is regularly spun out for a book column on a seasonal basis. And marginalia is a fascinating subject. Some book graffiti can be an art or an education in itself. Even when it’s rotten or wrong-headed, if it’s inspired by passion it has the power to take over the page.