X-link to the Non-Fiction megathread for another Christie-related work:
@ £0.99 from HarperCollins in the UK only,
Agatha Christie's Complete Secret Notebooks by Christie scholar John Curran is an omnibus edition of 2 books of his examining her writing and influences drawn from said notebooks which she originally planned and outlined her stories in.
This includes three previously unpublished (as of time of original publication circa the late 2000s) short stories: 2 Poirots and a Marple, plus some behind the scenes looks at the original manuscript ending for
A Mysterious Affair at Styles and other bonuses.
ETA: two high-quality companion guides are also discounted in the UK @ £1.49 & £1.99:
Agatha Christie’s Poirot: The Life and Times of Hercule Poirot by Order of Canada-recipient author Anne Hart &
The Life and Crimes of Agatha Christie: A biographical companion to the works of Agatha Christie by Charles Osborne (Estate-authorized author of the novelizations of her plays)