This doesn't happen so much lately, with a population falling over itself to jettison the physical manifestations of culture and abdicate the responsibility of rude possession to their overlords in the Cloud, our new Digital Olympus...but once upon a time people selling you books worried about the secrets you would glean from sorting through their library discards.
Not much, actually...or rather, nothing specific. I can tell if you are an interesting person, certainly- interesting people read interesting books. Long before we ever dated, Tiner brought in a bunch of books to sell before embarking for a year in the Czech Republic and remembers my effusive compliments on the selection. You can extract general suppositions from a selection of books, but without context you can't know what's actually going on.
(I amuse myself imagining the reluctant souls of yore happily loading up their Kindles from the Amazon site, thus inviting Jeff Bezos & his phalanx of data miners into their homes to construct a comprehensive, in depth dossier.)
But there are always exceptions to rules, like say you pick up a copy of Louise Hay's Love Yourself, Heal Your Life Workbook in a thrift store, not noticing that one of the chapters was filled with confessional answers in pencil.
And lemme tell ya, the type of parents who inspire their child to take up with Louise Hay will make you look pretty darn good in comparison. You can thank Louise's probing questions for these following tidbits-
Free Advice to Dads:
Spend more time with your kids and don't hit them!
Free Advice to Moms:
Tell your kids you love them and they are very special to you, don't hit them, and don't lie around sick on the couch all the time!