ANSWER: What makes a book a classic? (redux)

Originally, in our English tradition of Eurocentric scholarship, the term "classic" or "classical" referred to the great ages of Greece and Rome, and the literature they produced - think Homer, Sophocles, Cicero, Plutarch, etc. The term in this sense has come to be applied to the great works of historical eras of other areas of the globe.

As history went on, the term came to be used for work that endured from previous centuries, because (presumably) of qualities that keep people reading it - Chaucer, Shakespeare, Milton (to use English examples).

You can see some of these tensions and variations if you look at a dictionary definition, such as this one, or this one (you'll have to search the term "classic" in the latter).

If you can get to a college or large public library that has the Oxford English Dictionary or other large unabridged you can see how the term has been used, and changed over time, in various senses.

For better or worse, there is really no authority that controls and establishes criteria for terms like this, and so some people will apply the term very easily. Think of advertising, which is what a lot of such uses would be. Terms tend to get watered down, and some might say lose their meaning. For a discussion of how the term is applied to modern works (using The Color Purple as an example), see this previous Radical Reference answer.

Another resource that may help in understanding how the term is used is a quotation collection, such as this one or this one.