Which writer created Miss Marple and Hercule Poirot?

Agatha Christie, the best-selling novelist in history by a wide margin - something in the region of two billion copies, in more languages than any other author. She wrote 66 detective novels while also running a household and, during both wars, working in hospital dispensaries.

That dispensary work is why the poisons are right. Christie handled drugs professionally and knew dosages, symptoms and timings, and her books are accurate enough to have been cited in medical literature - a 1977 case in which a nurse recognised thallium poisoning in a child was credited to her having read The Pale Horse, and the same book is said to have helped identify a poisoning in another case.

She also disappeared for eleven days in 1926, after her mother's death and her husband asking for a divorce. Her car was found abandoned, over a thousand police officers and civilian volunteers searched, and she was eventually found at a Harrogate hotel registered under the surname of her husband's mistress, apparently with no memory of how she got there. She never explained it publicly and left it out of her autobiography entirely, and the episode is still argued about a century later.