I buy a lot of used books. One feature/problem with this is seeing the passages the previous owner(s) felt motivated to underline, scratch out, or otherwise highlight. Trying to determine their motivation for choosing that particular passage can be maddening. This is especially true of my copy of Kafka's The Trial. (e.g. this phrase marked with a faint line: "Everything lay in perfect order, but in his agitation he could not find at first the identification papers for which he was looking.")
I'd like to go through every used book I own, writing down each amplified passage. I think the combined phrases would make a great novel.