The Door by Mary Roberts Rinehart

The first time I heard of the mystery writer, Mary Roberts Rinehart was when our cruise ship stopped in Bar Harbor, Maine. After a tour of Cadillac Mountain, we stopped at a lovely tea room which was once part of the home where Mary Roberts Rinehard lived in the 1930s. Her 40th book, The Wall, was written there. It wasn't until we stopped at a book shop in Galveston many years later that I found one of her books.
The Door is a mystery that will keep you guessing until the very end. There are lots of suspects and twists and turns, but there is also a lot of repetition and I found the story moved slowly. Mary Roberts Rinehart has been compared to Agatha Christie who wrote during the same time period.

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