I write in books. I have always written in books, in pencil, and going back through the ones I have owned longest is an experience I would describe as roughly sixty percent useful and forty percent excruciating.
The three kinds of note I find
The underline with nothing attached. Useless, and about seventy percent of my marginalia. I underlined a sentence at twenty-four and I have no idea why. The past me had a thought; the past me did not write it down.
The agreement. “Yes.” “Exactly.” “!!” These are worse than useless — they are a record of the book confirming something I already believed, which is the least interesting thing a book can do and the thing I apparently most enjoyed.
The objection. The only kind worth anything. “But what about—”, “this contradicts p.41”, “he has assumed the thing he is proving”. Ten years later these are still legible as thinking, and about a third of them are better than the argument I would make now.
What I regret
Not the writing in books — I would do more of it. What I regret is the confidence. There are notes in my twenties-era books that are simply rude about writers who were, I now think, entirely right, and whose only crime was being harder than I was prepared for at the time.
The cure was not writing less. It was dating the notes. Once a marginal comment carries a year, you read it as a position held by a particular person at a particular time, and the whole thing becomes a conversation between versions of yourself instead of a verdict.
The practice now
Pencil, always. Objections in the margin, dated. Underlines only when accompanied by a word explaining the underline. And on a reread, the new notes go in a different corner of the page, so the layers stay separate — which is where most of the value in rereading actually shows up.