The significance of Poirot's first novel lies in introducing the character of Hercule Poirot. It sets the foundation for his unique detective style, with his little grey cells and fastidious mannerisms. This first novel also gives readers a taste of the complex mysteries he will solve in subsequent stories.
Well, Agatha Christie's first Poirot novel is 'The Mysterious Affair at Styles'. This book introduced the famous detective Hercule Poirot. It was set during World War I and had all the elements that would later become characteristic of Christie's work - a complex mystery, a diverse cast of suspects, and of course, Poirot's brilliant deductive reasoning. The story revolves around a murder in a country house, a classic setting for Christie's mysteries. Poirot, a Belgian refugee, uses his unique methods to solve the case, which includes observing the tiniest details and understanding the psychology of the people involved. It was a great start to what would become a long and successful series featuring this iconic detective.
I'm not sure specifically which one is Poirot's first novel. But generally, it might introduce the unique character of Poirot with his distinct deductive methods and quirky personality.