You may have notice that the Poirot reading list lists a short story called “Christmas Adventure” (1923). And you may wonder whether it refers to “The Adventure of the Christmas Pudding”? Fwiw, I copy and paste, here below, the answer I found at the Home of Agatha Christie website here.
Not quite. This is a slightly complex case. “The Christmas Adventure” is an early version of the story that would later be expanded upon and turned into the more detailed story “The Adventure of the Christmas Pudding.” However, “Christmas Pudding” is also published under the title “The Theft of the Royal Ruby.” “Pudding” and “Ruby” are pretty much the same story, aside from a possible changed word or two for cultural reasons between the US and UK versions. “Christmas Adventure” has never been published in an official collection in the US, although “Royal Ruby” is in Double Sin and Other Stories. If you want to read “Christmas Adventure,” you will have to go to a UK website or possibly Amazon.com and buy the UK collection While the Light Lasts and Other Stories (HarperCollins; Facsimile edition edition, 2011. Format: Hardcover. 224 pages. ISBN: 978-0002326438). Bottom line, “Christmas Adventure” has the same basic plot as the later versions, but it’s much shorter and some minor details are different from the later versions.
More from Wikipedia:
The Adventure of the Christmas Pudding is an expanded version of the story of the same name which appeared in issue 1611 of The Sketch magazine on 12 December 1923. The original shorter version was first printed in book form in the UK in the two obscure collections Problem at Pollensa Bay and Christmas Adventure (Todd 1943) and Poirot Knows the Murderer (Polybooks 1946) and was then eventually reprinted in book form in the UK collection While the Light Lasts and Other Stories in 1997 under the title Christmas Adventure. The expanded version appeared after publication of the book in the weekly magazine Women’s Illustrated from 24 December 1960 to 7 January 1961 under the alternative title of The Theft of the Royal Ruby with illustrations by Zelinksi. The story first appeared in the US in Double Sin and Other Stories in 1961 also under the title of The Theft of the Royal Ruby with some slight revisions to the UK version. The original shorter version has so far not been published in the US.