What great notes or marginalia have you come across? What are the types of notes you make in books?
On an aside, in my research I found that in the middle ages, monks used to fill their manuscripts with marginalia. These could be crude little asides, annoyance at the translation that was done before, or just mere exasperation at a tedious job. They would often illustrate their illuminated manuscripts too, again with jokes, or filthy asides to pass away the time hunched over a tallow candle in a drafty abbey somewhere on a moor. I was able to collect a large number of really weird medieval marginalia images, which I am sure I can share if you tweet the good people at Novlr for it.