Ultimately nothing. It all depends on the person or the book. The fact of being old doesn't mean you're smarter. Some people talk about "respecting the elders" 'cause they "lived and know a large amount if things", but most of the times all I can hear from them is a bunch of stereotypes about men, women, marriage and "respect". This does not mean one can't learn anything from, say, their grandparents. It all depends on who the person one's talking to is. But sometimes one can find way more wisdom in younger people or, as the question says, in books. Every type of book, but also movies and tv shows, can be way more effective. The stories they portray, be them historical or fantastic, may actually be way more useful to the spectator.
They may give the spectator an insight on the way people lived in a specific time / place, or they can help the spectator understand how they feel about something. Books and movies can also help people empathise, initially with the characters and then with people in the real world.
So, resuming, the answer to the question is "nothing", as everything is highly dependent on the person / book / movie one is talking to / reading / watching.