If you do there is a word for it, like there are words for every little action we habituate. The word is Marginalia ..
“When you buy a book, you establish a property right in it, just as you do in clothes or furniture when you buy and pay for them. But the act of purchase is actually only the prelude to possession in the case of a book. Full ownership of a book only comes when you have made it a part of yourself, and the best way to make yourself a part of it — which comes to the same thing — is by writing in it.” ~ How to Read a Book (1940) by Mortimer J. Adler.