When Agatha Christie published The Body in the Library in 1942, she was already a household name. Perhaps that's why she was confident enough to engage in a little self-parody.
When Mrs. Bantry wakes Colonel Bantry one morning by telling him the maid has found a body in the library, the Colonel replies, "You've been dreaming, Dolly. It's that detective story you were reading---The Clue of the Broken Match. You know, Lord Edgbaston finds a beautiful blonde dead on the library hearthrug. Bodies are always being found in libraries in books. I've never known a case in real life." Christie seems to be warning the reader that the premise of the book is typical for her mysteries set in English country houses.
Christie throws in quips about her profession throughout the book. When Miss Marple wonders who is calling her so early in the morning, the narrator recalls, "It was true that Miss Marple's nephew, a writer, and therefore erratic, had been known to ring up at the most peculiar times." No doubt she was replying to popular misconceptions about writers being oddballs.
And when one of the prime suspects meets Miss Marple and hears she "knows all about crime," he asks, "'Do you---er---write detective stories?' The most unlikely people, he know, wrote detective stories. And Miss Marple, in her old-fashioned spinster's clothes, looked a singularly unlikely person." Miss Marple, who will prove to be a genius at solving the crime, replies, "Oh, no, I'm not clever enough for that." Here she reminds me of Flannery O'Connor, who said, "Everyone knows what a story is, until they try to write one."