Via Kahunna, an essay called Always Carry A Pencil:

“An adept reader “phrases” a book as Ella Fitzgerald “phrases” Cole Porter, here leaning into the words and holding them back, there partnering them as Kafka partnered Goethe in February 1912: “I read sentences of Goethe’s as though my whole body were running down the stresses.”

I could never sell my textbooks back to the campus bookstore because I’d “defiled” them with pen and pencil marks. (And my handwriting is nothing to be proud of.) I love getting a used book in the mail and finding somebody’s already written their comments in the margin, even if–and this is often the case–they’re just parroting their teacher’s words during a lecture. Note-taking in the margin is this wonderful thing far different from taking notes on a piece of paper. Papers get lost or separated from their context. Marginalia is always there. I’ll always remember what I was thinking or the connections I made when rereading old favorites, because the notes are right there in front of me. By writing what I think–by interacting with the text–the book becomes mine.

Also from Always Carry A Pencil, an excerpt from Billy Collins’ Marginalia:

Sometimes the notes are ferocious,
skirmishes against the author
raging along the borders of every page
in tiny black script.
If I could just get my hands on you,
Kierkegaard, or Conor Cruise O’Brien,
they seem to say,
I would bolt the door and beat some logic into your head.

Other comments are more offhand, dismissive –
“Nonsense.” “Please!” “HA!!” –
that kind of thing.
I remember once looking up from my reading,
my thumb as a bookmark,
trying to imagine what the person must look like
who wrote “Don’t be a ninny”
alongside a paragraph in The Life of Emily Dickinson.