A classic is a novel that represents a genre or a writing style, or it can be a novel that makes a contribution to literature. There are all kinds of classics, from horror classics to romance classics, from novels that sold millions of copies to a novel that changed how a genre was written.
“Vintage” is another term often associated with books, and these are typically considered books more than 50 years old but less than 100. “Vintage books” tends to apply to collectibles, comics and mid-century volumes prized for their nostalgic value.