Every breath you take - Michelle Spring

1st in a series of crime novels feauturing private eye Laura Principal. Laura and Helen share a weekend retreat in Norfolk and are reluctant to disturb the close friendship and sense of security they have enjoyed for many years. So when Helen suggests inviting the eager to please but unconventional Monica to share the cottage, Laura is uneasy. Then Monica is brutally murdered, and Laura finds herself drawn into an investigation which puts her own life in danger. As she unravels the secrets of Monica's life, the killer gets closer and closer and when Laura uncovers the final mystery, she realises that it is not only her own life that is threatened.