Armstrong was pure 100% white bread. Well-bred, middle class if not upper middle class, traditional in virtually every respect, her forte was gently undermining the kind of women's fiction you found in the slicks of the 1940s and 1950s. (I've always remembered how she challenged the masculinty of a girl friend's lover. "He's the sort of man who's interested in women's hats." An her own lover says: "Lord." She was also good at spoofing the Martha Stewarts of her day. You ciould tell what she thought of a woman just by how she set her table. Too fancy was deadly.)
Her fiction is...odd. Nearly everybody in her stories is neurotic and overmuch. My favorite Armstrong is Michief, a short novel that made a much-denigrated film called Don't Bother To Knock, which features chilling performance by a young Marilyn Monroe as a mentally unbalanced babsyitter. It's a flawed movie but for me an entertaining one.
Her greatest success was with her novel The Unsuspected which became a smash hit with Claude Raines. The problem with the film is that running time doesn't permit all the really slick plot twists Armstrong brought to the novel.
She died way too young, at sixty-four, at the heighth of her popularity. Her stories were regularly adapted for TV. She won the Edgar for her novel A Dram of Poison which again struck me as an...odd book. A clever book, a well written book, but one that always left me cold.
You see her at her best, I think, in her short stories, many of which are stunning. And you have to applaud the slick magazine editors of the time for publishing some of them. She published two collections during her lifetime and you won't find a bad one in the bunch. And a few of them are stunning, dark as noir but played out against middle class setting and situations. Even most of her cozier material has an edge (with one goofy exception).
I don't think she was nearly as good as Margaret Millar, whom she resembles in some way, nor Elizabeth Sanxay Holding, whom she also resembles, but she is certainly worth buying from used stores or the internet.