Bargain @ $1.99 each for today only (May 12th) from HarperCollins in Canada & the US as part of the Kindle Daily Deal (price-matched at Kobo CA, but not US at the moment; may or may not also be matched in other stores):
25 random novels by
Agatha Christie, including both series and standalones, as well as the first posthumous Poirot series continuation written by Sophie Hannah. Out of the ones I recall reading, I especially recommend
Peril at End House and
Cards on the Table, which are pretty good Poirots, IMHO, and IIRC,
The ABC Murders also seemed better-than-average.
Death in the Clouds ISTR as pretty meh, although it does have a clever means, and you may be interested in the depiction of vintage air travel.
Linkage for the lot at
Amazon.com,
Amazon.ca, and
Kobo CA
NB: Although Poirot #1,
A Mysterious Affair at Styles, is public domain in the US (but not in Canada), the HarperCollins edition contains restored text found in the Christie archives, which provides some sort of alternate post-case ending, plus has a special introduction by a Christie scholar.