"The couple at table six" by Daniel Hurst

An audio book.

Hannah (her PoV) is a waitress, about 40. She's seen the Murphys at table 6 for 6 months each Friday. They're about 45.

Nadine (her PoV) is Mrs Murphy. They have a girl, 21 and a boy, Adam. 18. Max (Mr Murphy) is a self-made millionnaire. He doesn't know it but Adam isn't his son. Nadine had a short fling with Kieran. Out of the blue Kieran contacts Adam. Nadine tries to negotiate with him, offering a million pounds for his silence.

Hannah doesn't have much of a life. A decade before she was engaged, but her boyfriend was unfaithful. She likes talking to clients. She had an affair with one (a married man) 2 years before, and hoped to elope with him. She ended up killing both of the Spinners. She's been secretly living in their house ever since.

Nadine is home alone when Keiran visits. He's changed his terms. She knifes him, phoning Max to say that she's killed an intruder. He's at Hannah's cafe. She follows him home in a taxi and sees him take the body away. She blackmails Nadine so she can have Max. She feels sorry for Max because she's been betrayed too. They offer her £1 million but she wants Max. Max arranges to come to her house in the night. They have sex. She tells him about Nadine's affair. The house-owner arrives, so they have to leave.

When Hannah visits Nadine and Max's house, she discovers that Nadine's been killed and Max is hanged. There's a suicide message explaining that Max had an affair with Mrs Spinner, killed them, and used their house as a love-nest.

In the epilogue we learn that a lonely regular customer who likes Hannah has killed Nadine and Max, writing the letter. He asks Hannah out.

The characters aren't affected by becoming murderers. It's crazy (though it suits the plot) for Hannah to stay at the Spinner's house. The vocabulary doesn't always match the characters and situation - "I struggled to discern anything"; "The first hint of sunlight was cresting on the horizon"; "residual paranoia", etc.