The Murder on the Links by Agatha Christie (1923)

The second Poirot novel and third Agatha Christie overall, The Murder on the Links takes place in France, showing some knowledge of the culture and society of that country in the '20s. (Although Christie's novels tend to take place in a similar series of upper-middle-class rooms, be they French or Ancient Egyptian.) A millionaire with interests in South America is found stabbed to death, in an open grave on the golf course at his villa. Suspects include his wife, son, private secretary, and a mother and daughter who live in the neighbouring villa. Poirot becomes involved when he receives too late a letter from the dead man, pleading for his assistance to protect him from an assailant.

The plot of this one, as critics tend to note, is extremely convoluted. A contemporary review accused it of making the brain reel, and Christie connoisseur Robert Barnard called it an instantly forgettable instance of ingenuity triumphing over common sense.

Although the narrative is expertly structured, leading you to believe this thing at that time only to then upset the knowledge and complicate the story further, it depends on a lot of coincidences. You don't expect realism from Golden Age mysteries, but you do expect some sense that the events might have happened. Only the hand of God or a novelist could have orchestrated the number of coincidences necessary to make The Murder on the Links play out as it does. 

That said, although it does indeed make the brain reel, you're left certain of who everyone is and why they did what they did by the end. That's where Christie's genius often lay. The plots might be incredibly complicated in their physical details but, motivationally, they rested on astute and simple premises.

Christie didn't really do atmosphere in her early novels, preferring a lightness of touch suitable for enjoyment on trains and in deck chairs. She never lost that lightness of touch, though later novels made a bit more of an effort in the way of suspense and tone.

You can see the seed of her later style here, however, with a second body arriving at an opportune time towards the end of the second act to maintain reader interest. There's even a couple of action set-pieces near the end, including Poirot's Watson, Capt Hastings, helping a beautiful woman escape justice because of course he does. Perhaps realising that Hastings, a profoundly dense and silly character, would try her reader's patience if allowed free reign too long, Christie brings his romantic adventures to an end in this second story.