This book made the WSJ's list of best mysteries for 2022. I don't always find the WSJ reviewer a reliable guide, but he was on target here.
I remembered, dimly, that famous mystery writer Agatha Christie went AWOL back in the 1920s, accompanied by a great deal of public speculation. She returned after a few days, but her whereabouts and activities during that period remained mysteriously unexplained. This book weaves a tale around that incident and, although it's been at least forty years since I read a Christie novel, I detected (heh) hints of her plotting style here.
The narrator is Nan O'Dea, and she's a self-admitted homewrecker. She has aimed herself at Agatha's husband, Archie; as the book opens, she's successfully convinced him to divorce Agatha. But it's pretty clear from Nan's narration that Archie is not only an unfaithful cad, he's also kind of a dimwit. Nan clearly has ulterior motives, but what are they? They are eventually revealed via horrific flashbacks to her unhappy youth in very Catholic Ireland.
Another layer to the mystery is added when Agatha vanishes. A frantic countrywide search ensues. A cop, Frank Chilton, is pulled out of retirement to help out with that; he becomes a major character in the narrative.
And a couple of poisoning deaths occur along the way. And Chilton needs to check those out too. (And he's a nice enough guy, but he's no Hercule Poirot.)
A final mystery for the reader: Nan is narrating events, but just how reliable a narrator is she? Especially when she's describing things she didn't actually witness? How much of this story is told through speculation, delusion, lies, and maybe a touch of insanity? No spoilers here, but it's something to keep your eye on.