Anthony Gilbert 
The Fingerprint [EPUB ebook] 

A crime that isn’t reported; a mother who’s threatened… Classic crime from one of the greats of the Detection Club Out walking in the snow with her young son, Mike, Sara Drew is the only witness to a particularly unpleasant car accident in which an elderly lady and her dog are killed. Nervous of going to the scene of the accident with a child, she reluctantly goes home. There she meets one of the drivers, and is appalled that he has no intention of reporting the incident. And he also threatens to hurt her son if she goes to the police. But when Mike is kidnapped she contacts Detective Arthur Crook, who takes over the case and counters the wiles of desperate criminals . . .