I Let You Go by Clare Mackintosh

One of The New York Times Book Review’s 10 Best Crime Novels of 2016!

Now in paperback—the next blockbuster thriller for those who loved The Girl on the Train and Gone Girl... “a finely-crafted novel with a killer twist” (#1 New York Times bestselling author Paula Hawkins).
 
On a rainy afternoon, a mother's life is shattered as her son slips from her grip and runs into the street...
 
I Let You Go follows Jenna Gray as she moves to a ramshackle cottage on the remote Welsh coast, trying to escape the memory of the car accident that plays again and again in her mind and desperate to heal from the loss of her child and the rest of her painful past.
 
At the same time, the novel tracks the pair of Bristol police investigators trying to get to the bottom of this hit-and-run. As they chase down one hopeless lead after another, they find themselves as drawn to each other as they are to the frustrating, twist-filled case before them.