I agree. If I think something is significant or interesting, I'll use small post-it/sticky notes. I had a lot if stickies in Gravity's Rainbow!If the book isn't yours it's vandalism and I won't buy a second hand book with some one else's notes in it.
The only books I write in are text books and then mostly not. Even if I thought a thing important once, does not mean I will still think it important later. I prefer using a notebook.
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