I think, about the distinction between fiction and nonfiction. Fiction is not really about anything: it is what it is. But nonfiction - and you see this particularly with something like the BBC Samuel Johnson Prize for Non-Fiction - nonfiction we define in relation to what it's about. So, Stalingrad by Antony Beevor. It's "about" Stalingrad. Or, here's a book by Claire Tomalin: it's "about" Charles Dickens.