Ancient Worlds: What Kind Of Book Is This, Anyway?

We all know the old adage “Never judge a book by its cover.” But when it comes to literal application, we all do. That’s because our book culture encourages it. The cover won’t always tell us much about the quality of a book, true, but if we want to know what kind of book it is, the cover is where we find out.
Shirtless guy holding woman in historically inaccurate clothing in highly improbable position? Romance.
Shirtless guy flexing while holding sword (with or without scantily clad woman clinging to his knees)? Heroic fantasy.
Woman scantily clad while still wearing a lot of black leather with a sword, possibly straddling a motorcycle? Urban fantasy. Probably.
Genre is how the publisher knows what kind of cover to put on a book. It’s also how the reader knows, more or less, what to expect when they pick a novel up. If you buy a book with a silhouette of a tank on the cover and get a story that is 90% romance, you’re going to be perplexed.
The same was true of ancient books, although the cues were given in different ways.