The Keys To The Street by Ruth Rendell

Mary Jago donates her bone marrow to save the life of a complete stranger; a generous act of kindness that culminates in a violent break-up with her brutish boyfriend.

Moving to the affluent edge of London's famous Regent's Park, Mary believed she had finally escaped the threat of violence. She never thought that one simple act of kindness could put her own life in mortal danger.

When the bodies of local homeless people are found impaled on the park's railings, violently murdered by a deranged serial killer, Mary could not have suspected a connection to herself. But on the dark and mysterious streets of Rendell's labyrinthine London, everyone is trapped in her tightly woven web of murder and mystery.