Occasionally during the 'Doctor Who' episode "The Unicorn & The Wasp", characters would invoke the eternal question, "WWPD?": "What would Poirot do?"
Yes, that's right. I'm STILL not done with this episode and its contributions to the Toobworld mythos!
Agatha Christie was complimented on her books about Hercule Poirot, and she referred to his signature phrase regarding his "little gray cells".
But no one, not even Mrs. Christie, actually stated that she created Hercule Poirot as a fictional character. Which is a good thing, because Hercule Poirot and Agatha Christie both exist in Toobworld!
Agatha Christie wrote stories about the little Belgian detective, but she was serving almost as his biographer in a way. She treated Poirot and his cases in much the same way as Truman Capote did with the killers of "In Cold Blood": she was writing factual stories about him.
Later, when Donna Noble accidentally suggested Miss Marple and her way of detection to Agatha Christie, the author thought it would make for a great story. But this doesn't mean that Mrs. Christie created Miss Jane Marple either. She would later find out that Miss Marple actually did exist, and so she decided to work out a contract arrangement between the two of them. Her books about the old lady of St. Mary's Mead would also turn out to be factual novels.
[We see pictured here two of the actresses who played Miss Marple on TV. Joan Hickson, on the left, is the Jane Marple of Earth Prime-Time. Geraldine McEwan on the right would live in the village of St. Mary's Mead as well, but in an alternate TV dimension, perhaps the one of TV series remakes.]
Agatha Christie then falls into the same Toobworld category as authors like Mark Twain, Jules Verne, Charles Dickens, and Stephen King (who appeared as a little boy in an episode of 'Quantum Leap'). That is, she shares the same universe as the characters she created. (And this would include Ariadne Oliver and Tommy and Tuppence as well.)