Based on a true story

The sign says in large friendly type:
"HARDCOVER PROMOTION - BUY 1 FICTION, GET 1 FICTION FREE."

So after some browsing, I grab two books I like and fish 20 bucks out of my wallet.

Cashier: 36 please.
Me: 36? Isn't it 18? Buy one, one free right?
Cashier: Only for hardcover fiction, sir.
Me: But they're both hardcover.
Cashier: Yes, but only one is fiction.
Me: But they're both fiction.
Cashier: Only this one is fiction (speaks as if to a mentally retarded child).
Me: As opposed to this one (points to book) a true story of vampires attacking a small town in Maine?
Cashier: Only this one is fiction, sir (obstinate now).
Me: And this one?
Cashier: Horror.

And he was right. It was. But not the book.