A Good Rest for Some

Good Rest Care Home, “A luxury converted manor house where residents are well looked after by experienced, caring and compassionate staff. Let your loved ones stay here in safety, in superbly decorated accommodation single en suite bedrooms are decorated to a high standard and residents are encouraged to bring in their own possessions to personalise their rooms.”
A country home where residents are safe... aren't they?
Things go badly wrong when residents start losing their possessions from their rooms. And when four residents die, the deaths are attributed to natural causes. But three members of staff are not convinced, so they undertake their own investigation.
With an unconcerned manager and a mysterious doctor, the three discover that the residents are not in safe hands and are in danger of dying before their time.
Who is the person or persons responsible? Can the three staff members find out before any more untimely deaths?