The more I read Poirot's adventures the more I love Poirot. And indirectly his friend Captain Hastings. Hastings doesn't feature in this story but Poirot still refers to him in the way that only Poirot can - with harsh endearment.
But onto the murder. As I've said Hastings isn't in this story so he isn't narrating it. But it still is first person narration, the narrator being the neighbour of Poirot. And so we see Poirot through another person's eyes, and enjoy another person's bewilderment at Poirot's eccentricities.
Other important plot background is that Poirot is meant to have retired, which is why he is living in the country next door to the village doctor. Who apparently reminds Poirot of Hastings. Until the murder takes place no one knows Poirot's true identity, apart from Roger Ackroyd who knew Poirot in the city. Poirot is upset by the murder of his friend and so lends assistance, which is just as well because if he didn't I'm not sure the murder would ever get solved!
If like me you have seen the TV adaptation of this you might be surprised by the ending of the book. For me the jury is still out on which is the more satisfying ending.