Agatha Christie's Secret Notebooks: Fifty Years of Mysteries in the Making is a book written by John Curran[1] and published by HarperCollins[2] on 6 September 2009, which later went on to win the Anthony Award for Best Critical Nonfiction in 2011.

[3] A sequel, titled Agatha Christie's Murder in the Making: More Stories and Secrets from Her Notebooks, was published by HarperCollins in 2012.

[5] The book includes two unpublished Hercule Poirot stories - a previous version of The Capture of Cerberus, and The Incident of the Dog's Ball.

The Capture of Cerberus was originally intended to be included in The Labours of Hercules as the final story, but was rejected due to the political content - set just before World War II, it involves Poirot solving the shooting of the German dictator, 'August Hertzlein'.

Returning to Britain, Poirot hires Mr Higgs, a dog thief, and, with a cat burglar, they travel to a place in Alsace.

Mr Higgs uses his skills to get the massive guard dog out of the way, while the cat burglar breaks into a certain room in the house, files the bars away, and with the aid of a silk ladder, lowers Hertzlein - a "short man with a bullet head and a little dark moustache" - to the ground.

"Executive authorities in the Central Empires" kidnapped him and substituted a look-alike, who was then assassinated by two storm troopers who then accused Hans Lutzmann, forcing the gun into his hands.

Poirot learns that she made her will leaving everything to Miss Lawson after falling down the stairs, having supposedly slipped on Bob's ball.

Poirot sees a piece of needlework depicting a bulldog on the steps of a house, with the caption 'Out all night and no key!'.