Mary Roberts Rinehart is often considered as the American Agatha Christie. However, she was sixteen years older than Dame Agatha and began her writing career before her. Some people say she was the creator of "The butler did it" phrase but apparently she never used it. According to Wikipedia, "She is considered to have invented the "Had-I-But-Known" school of mystery writing."
My favorite novels by Rinehart are The Red Lamp and The Wall. Many years have passed since I read The Red Lamp and I had to research the novel to refresh my memory. It is a seemingly supernatural story, eerily suspenseful. I read The Wall in recent years and was impressed by its clever plot regarding the murder victim who was beautiful and devious and hated by apparently everyone. This past year I read The Bat, "a costumed super criminal", which, according to Wikipedia was one of the sources of inspiration for Batman although I couldn't see a connection at all.
In 1907, she wrote The Circular Staircase, the novel that made her famous. I remember reading it during a visit to Cuernavaca, Mexico many years ago. I must have found it in a bookstore that sold used books to tourists.
Luckily, I have not read all of her books and have many hours of pleasurable reading ahead of me.