A classic is a book accepted as being exemplary or particularly noteworthy. What makes a book "classic" is a concern that has occurred to various authors ranging from Italo Calvino to Mark Twain and the related questions of "Why Read the Classics?" and "What Is a Classic?" have been essayed by authors from different genres and eras (including Calvino, T. S. Eliot, Charles Augustin Sainte-Beuve). https://en.wikipedia.org/wiki/Classic_book