I’ve a complicated history with marginalia in books.
At my high school, teachers would skim through textbooks at the end of the year, checking to see if you’d written in the book. Underline too many things and they’d charge you for a new copy. For a working class kid like me, that was no small threat.
In college, my scholarship included a small allowance for books. It wasn’t enough to cover them all, so I bought used ones whenever I could. Some had been well used. I remember an organic chemistry book in which one particularly difficult chapter was marked entirely in varying shades of highlighter.
Buying books was a bit of a gamble.
Look for one with no markings and trust yourself to pick out what matters?
Find one with lots of marginal notes and hope the previous owner understood the material?
Buy books directly from friends who were smarter than I?
It’s a reminder that there are a lot of things you can do with a book.
Loan it to a friend.
Write in the margins.
Stick pieces of paper in between pages.
Use it to hold open a door.
These are a book’s paratextual elements (well, not that last one). They're the sorts of things that happen outside of the literal text of the story that lend themselves to the interpretation of the text.