A margin note preserves the reader you were before the next chapter.

Writing in a margin freezes a version of you that the rest of the book is about to alter. The question you scribbled on page forty was asked by someone who did not yet know what page ninety says, and that ignorance is worth keeping. Rereading your own notes later is less about the book than about meeting the person who made them.

Keep a pencil with whatever you are reading now and mark the places you disagree or do not follow. Date the note if you like. Years from now the value will be in how differently you would write it.