This is a Miss Marple book, so I’ve both read it before (at least twice) and probably seen a dozen adaptations of, but it didn’t seem that familiar to me, anyway, so I gave it a go.
But as soon as I started reading, I immediately remembered the central gag of the story. It’s still pretty entertaining, although Christie had to pad it out a bit by adding various cops talking to each other a bit too long. I had indeed totally forgotten who the killers were, and couldn’t re-figure it out, either.
It’s a fun plot executed well.
The Body in the Library (1942) by Agatha Christie (buy used, 3.81 on Goodreads)