It looks like 50ers are reading all sorts of stuff, so let me put in my two cents as a retired teacher of literature. For years I was a speaker for the Montana Committee for the Humanities. One of the programs I presented was entitled The Ten Greatest Novels and How to Read Them. The list changed over the years, but here are a bunch that you can't go wrong with though some might take a little time and energy. You might think that the list is a bit academic, which it is. All of these books appear on college reading lists somewhere or other, but all are great novels and accessible to serious readers who want to extend themselves beyond just popular novels, which are primarily escapist and entertaining (nothing wrong with escapism or entertainment, of course). These novels are entertaining as well, but stimulating and enlightenening in addition. Here goes:

Tolstoy: War and Peace; Anna Karenina
Dostoyevsky: Crime and Punishment
Stendahl: Red and Black
Flaubert: Madame Bovary
Dickens: David Copperfield
Conrad: Heart of Darkness
Gaskell: Wives and Daughters
Eliot: Middlemarch
James: Portrait of a Lady
Hemingway: A Farewell to Arms
Fitzegerald: The Great Gatsby
Faulkner: The Sound and the Fury
Twain: Adventures of Huckleberry Finn
Melville: Moby Dick
Hawthorne: The Scarlett Letter