What makes a classic and, more importantly, can one apply these variables to the books that are published today?
The former is consistently debated. Of course, there are extensive discussions on the matter, but there is a consensus about time needing to play a factor. On the other hand there is disagreement about whether it's the experts' or everyday readers' opinions which matter for a book's longevity.
What follows are some factors for what makes a classic:
-Addresses universal human concerns
-Shifts people's views on life
-Influences subsequent works
-Merit, which is continually respected and examined by experts and critics throughout the years
These matters are notable, because if we are to answer the second part of our initial question—picking the classics from modern books—it appears there's nothing for us to do but to wait for posterity to figure it out for us. This is the only definitive way to find out about a book’s long-term viability, but that's too easy an answer for me, a person who wants to read contemporary books and be able to figure out what will last.
What then should one look for in an excellent book, but without the luxury of being wrung through time? There are certainly other matters to consider.
The questions a novel asks should highlight elements that were not previously well-known by the Zeitgeist. For example, Things Fall Apart by Chinua Achebe is considered a classic by many as it was one of the first retorts from Africa to the otherwise Eurocentric literature concerned with the African continent. By providing the West African view of life during colonialism, Things Fall Apart is an apt contender for future classic status. Highlighting a less-often seen viewpoint or moment in time make a book stand out; it isn't merely adding to the echo chamber of popular literature that is discussed, let's say, in online forums. Being labeled “literary fiction” by a novel's contemporaries doesn't necessarily indicate its long-lasting appeal.
To speak to the contemporary human condition, a book must be able to reach into the disquiet that most people feel in society and explain it through prose. The disquiet in Things Fall Apart was the need for the African story to be told beyond the stereotypical image of savages in shadows, which so repulsed writers like Achebe. The disquiet here was about the need for an identity that wasn't forced by outsiders.
Rereading is an additional factor that is often overlooked, but one that should be heavily considered. If a book can take several reads without suffering and losing luster in the eyes of the reader—perhaps even gaining from it—then it surely can be marked as a book for the future. I think of 2666 by Roberto Bolaño. I would like to go back and relive some of what the characters went through, but to also mull over the philosophical thoughts Bolaño keenly presented in the book. I find that there have been many times when 2666 came back to me when I had thought it long forgotten.
For any book to warrant this, it must have characters with complexity, layers revealing surprising facets or a relationship to the plot that is unexpected and not banal, while also addressing the philosophical questions used to investigate the vicissitudes of humanity. In a way, this is like a statistical sampling of the definitive factors associated with the aforementioned time and posterity. We can shorten the length of one person's life and see if they are willing to read a book more than once. Does this book last through an individual's life? Any book that retains import through the many seasons of one person's lifetime can be considered for classic status. For to do so, it will need relevance and meaning that can outlast any fashion of the Time and, thus, remain in one's mind. Superficial and shallow trends that we tend to chase for short periods will not merit consideration. Many best sellers and page turners usually don't meet this requirement. They are enjoyable, but to want to reread them—and to want to do so over a lifetime—is another matter.
How can we show that these criteria will work, especially when we don’t know what turns society will take and what future readers will consider well-written? What could possibly be insightful for them? For now, we can look into the current crop of books and see which ones come closest to these criteria. Then you can predict or make fifty-year bets. Why not?