The Door — Mary Roberts Rinehart

How little you know about even the people who are closest to you...This is what ran through Elizabeth Bell's mind on the night of April 18th. Sarah Gittings, the family nurse, had just been brutally murdered. And all thoughts of a homicidal maniac running amuck were banished when the evidence revealed that Sarah had known and trusted her murderer...

Poor Sarah. Who would have anything against her? Who indeed. Elizabeth Bell was about to discover that the staid and orderly household she had presided over for years in peaceful isolation harbored more than one suspect with a motive. And, unfortunately, more than one victim...