Charles Dickens: After all these revelations, there’s one mystery you still haven’t explained. Answer me this. Who are you?

Doctor: Just a friend. Passing through.

Charles Dickens: But you have such knowledge of future times. I don’t wish to impose on you but I must ask you, my books… Doctor. Do they last?

Doctor: Oh, yes.

Charles Dickens: For how long?

Doctor: Forever.

From Doctor Who – Season 1 Episode 3: ‘The Unquiet Dead’ (1×03)