How to Take Smart Notes by Sönke Ahrens is a book about the Zettelkasten method, invented by a German sociologist named Niklas Luhmann who published more than 70 books partly because his note system did most of the work. I read it thinking I would get productivity tips. I got a small, quiet revolution in how I read.
The key idea, which is embarrassingly simple: never take a note that is only about the source. Every note should be a self-contained thought, written in your own words, that you could still understand ten years later. The corollary is that most highlights and margin scribbles are useless because they only make sense in context. If you cannot rewrite the idea in a sentence, you did not really absorb it.
I stopped highlighting after this book. I stopped taking notes during the reading and started taking them after — 24 hours after, in a separate session, without the book in front of me. The friction went up. The number of notes went down. And two years later I am actually still using half of them, which is more than I can say for the previous twenty years of highlighter marks.