Thirty-year old Mrs Agatha Christie turned a nice little profit with this, her first book, in 1920. It introduced Hercule Poirot. Wisely, she gave him many flamboyant, eccentric characteristics to leaven the depiction of detection work, but unwisely she created a character of advanced age that she subsequently needed to preserve for a further fifty years.
What became the regular Christie recipe for a whodunit is found here. Perhaps there is a tad more reliance on the dispensing of medicines, reflecting the author's occupation during World War One. A formula that she later discarded was the use of a narrator - Hastings - who presents one of the "cases" on his friend Poirot. 1920 and the publication of this book marked the opening of the "Golden Age of Detective Fiction". Expect that there are plenty of servants, plenty of drinks at bedtime, much making and re-making of wills, and characters - including Poirot - who walk everywhere.
This rates highly in the Christie collection for classic charm, readability and ingenuity. Few of her books from the 1920s excel it.