The only time it’s acceptable to write notes directly into a book is when you’re taking it into an exam. Otherwise, it’s just an act of vandalism!
Pictures if you want them: https://twitter.com/ZainRzv/status/1289964671563214848
Saying it’s vandalism seems a bit extreme to me—-unless we’re talking about a book borrowed from a friend or a library.
I don’t really write notes in margins much anyway. It never seemed useful to me to bind my notes directly to the pages that they come from. They become too hard to find.
Now though, I'm delighted to write notes all over
I’m only really calling it vandalism with my tongue firmly in cheek–obviously anyone can do what they like with their own books :)