Severus writes in almost every book, but it is to an extreme in potion books. Almost in every available spot are notes. He even changes amounts in recipes, probably perfecting it after a lot of trial and error. I have to admit to be also fascinated with his way of thinking, it is very similar to mine, or other way around I guess. Still, I would not write inside a book, I would prefer to use sticky notes, that way I can even use a color code.