The Book Haven blog has the writer Italo Calvino’s list of what makes a book a classic:
3. The classics are books which exercise a particular influence, both when they imprint themselves on our imagination as unforgettable, and when they hide in the layers of memory disguised as the individual’s or the collective unconscious.
4. A classic is a book which with each rereading offers as much of a sense of discovery as the first reading.
5. A classic is a book which even when we read it for the first time gives the sense of rereading something we have read before.