What Makes a Book A Classic?

[Wonky]

What makes a book a classic? That’s one of the most acrimonious, endless and irresolvable discussions in the literary world. Like debates over which books are “great” (and why), it’s also a mostly pointless question, fodder for overcaffeinated undergraduate bull sessions, feral comments threads and other milieus suffering under the delusion that we can arrive at an ironclad consensus on what constitutes literary merit."

More at:
http://www.salon.com/2014/01/30/what_mak...a_classic/

Anyone here have a view of what makes a book a classic?