From The Guardian:
[T]wo previously unpublished Hercule Poirot stories have emerged from a mass of family papers at Agatha Christie's favourite home.
The first story, The Mystery of the Dog's Ball, eventually became the 1937 novel Dumb Witness, in which an heiress dies from falling down the stairs after apparently tripping over her fox terrier's toy. The title of the other new find, The Capture of Cerberus, has graced another story. The original was written to complete The Labours of Hercules, a collection of Poirot's 12 last cases. The first 11 were published in the Strand magazine between 1939-40, but the last only appeared in the book published in 1947 – a new story keeping only the title from the notebook version.
They will appear in Agatha Christie's Secret Notebooks: Fifty Years of Mysteries in the Making, to be published by HarperCollins in September.