At their very best, a good book can change a person's life. Like all forms of media, this doesn't just have to be because of the thing itself. The stories, meanings, and interpretations that we build around them can be just as important, if not more so.
It would seem that this is an idea that is well understood by a secondhand bookstore stumbled upon by @looks_last, who claimed that the business taped the notes and other ephemera that they find to a wall in the store. What followed was a charming, entertaining, and occasionally bittersweet thread in which people detailed all the different kinds of things which they had found in used books.
Before the advent of social media, it was a rare occurrence for us to get an intimate glimpse into the personal lives of strangers. However, this goes to show that it wasn't unheard of, in ways that still happen today.