The Safe House by Nicci French

Samantha Laschen, a doctor specialising in post-traumatic stress disorder, has moved to the Essex coast with her small daughter, Elsie, to escape the problems of her London life. Or so she thinks. Fiona Mackenzie barely survived the savage, murderous attack which left her parents dead. Now she is in need of sanctuary and the police see Sam as the ideal person to offer her a safe house. But as Fiona makes her way into the hearts of her hosts, Sam discovers that the risks she has foreseen are nothing to the terrifying danger she actually faces ...