Third Girl

 Third Girl by Agatha Christie (1966) 218 pages

In this Hercule Poirot mystery, a young woman‒with vacant eyes and unattractive hair and clothing‒comes to Poirot to say that she may have committed a murder. He starts asking questions, but she stammers and soon leaves, telling him she's sorry, but he's too old. Thus insulted, Poirot is mulling this over when his old friend Ariadne Oliver calls. It turns out that the young woman fits the description of someone Mrs. Oliver recently met named Norma. Mrs. Oliver happened to talk about Poirot in a conversation with Norma.

Norma and her mother had been abandoned by Norma's father 15 years ago when Norma was five. After Norma's mother died,  Norma's father reappeared with a new wife to run the prosperous company that his now-deceased brother owned. Norma doesn't get along well with her stepmother, and takes a room in an apartment with two other young women, including one who works for Norma's father. Norma is involved with a young man who just doesn't seem suitable. Is he really interested in her or is he out for her eventual inheritance? As facts come out, Poirot and Mrs. Oliver (and a private investigator that Poirot has hired) try to figure out why Norma thinks she might have murdered someone and who it might be.

This is a classic Christie, one that I'd first read years ago. As Poirot often says, once the puzzle pieces are there, one needs to work to rearrange them to see the truth.