A margin note can become a conversation between people reading decades apart.

A note in a margin can be a conversation across decades. Someone argued with a page, and another reader arrives later and argues back, and the two of them are engaged with each other without any chance of meeting. Books turn out to be one of the slower forms of conversation.

So write in your books, if they are yours. Marking what you disagreed with, what you did not understand, what you want to return to makes reading active and leaves something for whoever comes next. And when you find someone else’s notes, read them. They are usually more interesting than the sections everyone underlines.