Rather than try define it, would it be easier to allow Marketing to determine it? That is to say, "Literature" is that which you'd expect to find across major pbook chains in their section, for those that have them, called "Literature" (with "Classics" likely included, as well as perhaps "Plays" and "Poetry", as desired). That would seem to meet initial expectations and desires of this Club, and perhaps the inspiration (memory of such "Literature"-browsing) behind its creation.
Too much discussion eventually ratchets out parameters to a universal satisfaction such that initial desires get smeared into a generic "everything". Hardly anyone ends up happy, especially those who initially ran with the idea before it changed.
Respect the wishes of the initial proposal and proposer. Be arbitrary if it short-circuits endless discussion and stops the doing. And if someone doesn't like it they can start their own Club, no harm done.
But that's just me, contributing his own "endless discussion".