From Why Read the Classics by Italo Calvino (1923-1985). This list is only a small portion of an essay worth reading in its entirety:
1. The classics are the books of which we usually hear people say, "I am rereading . . . " and never "I am reading . . . "
2. We use the words "classics" for books that are treasured by those who have read and loved them; but they are treasured no less by those who have the luck to read them for the first time in the best conditions to enjoy them
3. The classics are books that exert a peculiar influence, both when they refuse to be eradicated from the mind and when they conceal themselves in the folds of memory, camouflaging themselves as the collective or individual unconscious.
4. Every rereading of a classic is as much a voyage of discovery as the first reading.
5. Every reading of a classic is in fact a rereading.
6. A classic is a book that has never finished saying what it has to say.
7. The classics are the books that come down to us bearing the traces of readings previous to ours, and bringing in their wake the traces they themselves have left on the culture or cultures they have passed through (or, more simply, on language and customs).
8. A classic does not necessarily teach us anything we did not know before. In a classic we sometimes discover something we have always known (or thought we knew), but without knowing that this author said it first, or at least is associated with it in a special way. And this, too, is a surprise that gives much pleasure, such as we always gain from the discovery of an origin, a relationship, an affinity.
9. The classics are books which, upon reading, we find even fresher, more unexpected, and more marvelous than we had thought from hearing about them.
10. We use the word "classic" of a book that takes the form of an equivalent to the universe, on a level with the ancient talismans. With this definition we are approaching the idea of the "total book," as Mallarmé conceived of it.
11. Your classic author is the one you cannot feel indifferent to, who helps you to define yourself in relation to him, even in dispute with him.
12. A classic is a book that comes before other classics; but anyone who has read the others first, and then reads this one, instantly recognizes its place in the family tree.
13. A classic is something that tends to relegate the concerns of the moment to the status of background noise, but at the same time this background noise is something we cannot do without.
14. A classic is something that persists as a background noise even when the most incompatible momentary concerns are in control of the situation.