Ordeal by Innocence is a standalone novel written in the late 1950s and set in the same period.

Arthur Calgary returns from a polar expedition and realises that he is the alibi for a convicted murderer who died in prison. He tells the family of the dead man, thinking they would be relieved that their brother/son has been exonerated. However, instead, it throws everyone else in the family into suspicion for the murder of their mother.

I found this, like And Then There Were None, to be more psychologically thrilling with more of a sense of menace than her usual cosy murder mystery formats. It was well-plotted and kept me guessing with a good resolution to the mystery. My only bug-bear were the two romantic relationships between much older men and young women. Not that I don't understand how they occurred and it was likely more common and isn't that icky given they're both adult women, but still irked me slightly.