" Okay, what’s fictional nonfiction, then?” I asked.

“Same idea. Not all the fictional books are fiction,” explained Dr. Rust. “Some are nonfiction.”

“Huh?” Now I was thoroughly confused.

“Oh, for example . . .” Dr. Rust hesitated.

Elizabeth suggested, “The Key to All Mythologies?”

“Yes! Good one. That’s in Middlemarch, a novel by George Eliot. A fussy scholar spends his life writing it—The Key to All Mythologies, I mean. It’s nonfiction, but it exists in the novel, which is fiction. So it’s fictional nonfiction. See? "

― Polly Shulman , The Poe Estate (The Grimm Legacy, #3)