Ever considered why a book or any work has staying power?
A classic book is one that out-lasts time. So it not only spreads in space through word of mouth epidemic at a specific point in time, it also spreads forward into the future.
For this to happen it has some key elements that make it meaningful, regardless of the century in which it is read.
Some key things to think about:
Relevance – Classic books are highly relevant to any period. 30 years into the future, people must still understand what you were talking about. Humor or discourse that is only relevant to the period, can stop e a books from becoming a classic.
Morality – Classic books typically have a profound moral lesson. A simple one relevant to all time has the best staying power. Issues that have staying power are typically aligned with fundamental human rights – freedom, self expression, happiness etc.
Quality – Classic books are well written. They hook the reader and keep them hooked.
Credibility – Does your reader believe what you’ve written. Or in the case of fantasy of science fiction, have you succeeded in having them suspend their disbelief? A book must be well enough researched that readers do not get annoyed with implausible fact and situations.