4 ideas
Diana Cowper, a wealthy London widow and mother of a rising actor, walks into an undertaker's and arranges every detail of her own funeral, then is strangled in her home a few hours later. The investigation leads back ten years to a road in Kent where she ran down twin boys, killing one and leaving the other brain-damaged, and escaped serious punishment.
Horowitz writes himself into the novel under his own name, with his real career, projects and colleagues, as the chronicler of a detective he doesn't fully understand or like. Because the narrator really does write the reader's text, he can record every clue faithfully while visibly misreading it. That keeps the puzzle fair: the reader gets all the data but inherits the narrator's wrong interpretations.
The investigator starts not from who had motive but from the one fact that doesn't fit normal behaviour, here a healthy woman booking her own funeral on the day she dies. Asking why that anomaly happened, and what the victim knew or feared, narrows the field faster than listing suspects. Choices inside the anomaly, such as the music and readings she picked, carry information about her state of mind.
When a court leaves a grievous harm lightly punished, the grievance doesn't end. It moves to the victims' circle and can wait for years.