I have a big personal interest in this subject as well. When younger my friends and I used to exchange books and novels from our personal libraries to read. If something I read gave me an idea for a comment, an observation, a poem, a song, a story, an invention or anything else of value then I would write in the margins I was reading.
At first my friends got angry about this practice then later on they started giving me more and more books than I was giving them. When I asked why they would often tell me, “what you write in the books is a lot of times more interesting than the book itself. I just wanted to read what you’d write in it when you read it.”
So I can personally verify this observation:
“In getting my books, I have been always solicitous of an ample margin; this not so much through any love of the thing in itself, however agreeable, as for the facility it affords me of penciling suggested thoughts, agreements and differences of opinion, or brief critical comments in general. Where what I have to note is too much to be included within the narrow limits of a margin, I commit it to a slip of paper, and deposit it between the leaves; taking care to secure it by an imperceptible portion of gum tragacanth paste … All this may be whim; it may be not only a very hackneyed, but a very idle practice; — yet I persist in it still; and it affords me pleasure; which is profit.”