I got Hayek's The Road to Serfdom out of the library and found it filled to the brim with scribblings. I estimate about 85% of the text is underlined in pencil, presumably to draw attention to the important parts.
Is this a social science thing? Maths textbooks don't attract notes in the margin, which could be an act of pan-disciplinary caution lest another Fermat come along and find the margins crammed with the dribblings of people who seem to think they aren't vandals.
Whoever these notemakers are, I've never met them before in my life, but they seem to me like slackers.