This novel is the second new Poirot story written by Sophie Hannah. I haven't read Hannah's other works, but I have read The Monogram Murders, the first novel she wrote with Poirot. I don't know if it's a feature of her work in general, but those two novels share many inelegant commonalities: the plot device of inauthentic, contrived conversations that are conveniently overheard and misunderstood; chapters that ramble on about some event in the past that somehow set up the current situation; and epicycles upon epicycles of complications that contribute nothing beyond padding out the book length and artificially obscuring the mystery's solution.
The writing in the book is fine: unremarkable, but not terribly distracting. Christie's dialogue was workmanlike but had a parsimonious efficiency to it. Here, characters just ramble on uninterestingly. The writing outside dialogue isn't much better. Catchpool is apparently psychic. When he's in conversations, we're told that his interlocutors speak as if they're concealing a secret, or that they're boiling with rage, or have some other emotion or concern. Yet there's nothing in the conversation itself to indicate that fact, and the reader isn't given any physical description or actions to corroborate it. We're just flat-out told, rather than shown, what people think. That's poor writing in general, but it's damning in a mystery novel. The reader should be drawing his own conclusions about the characters; instead, we're dependent on Catchpool not just for the facts of the case but also their interpretation.
The mystery itself has some compelling points. Without revealing any spoilers, you can clearly tell what part of the solution was the hook the author set for the story, and it's indeed worth basing a novel on (which I would not have said about the scenario in its predecessor). It's a clever setup, although its execution is not worthy of it.
I gave up trying to solve the mystery about halfway through. I lost confidence in the author's ability to tell a compact, self-contained story without a revelation three-quarters of the way through the book that some of the characters had a heretofore unrevealed secret backstory with the murder victim. That happened in the predecessor to this novel, and it happened in the current one as well. A murder mystery is supposed to be a puzzle for the reader to solve. With Catchpool's magical insights spoiling the current investigation and the unknowable backstory spoiling the murder itself, it's not a fair game. Instead of solving the mystery, I just patiently waited for the ending for the author to pull some arbitrary solution out of her hat. That solution had some gaping holes in it (for example, without introducing spoilers, the murder has no reason to be carrying around the murder weapon, and the novel lampshades this fact), but it more or less makes sense. It's not completely satisfying, but it's a definite improvement over the solution of the Monogram Murders.
So, how is the book overall?
As a reincarnation of Poirot, it feels clunky and fake. There's no real need for Poirot to even be in the novel; what little characterization occurs is mostly of Catchpool, the Hastings substitute.
As a reincarnation of Agatha Christie, it's just not up to the task. It feels like an ersatz knockoff: someone who's copied a few of the mannerisms but missed the whole point of the writing.
As a revival of a Golden-Age-style murder mystery, it's a respectable effort. It's not great, but I didn't feel cheated after reading it. It's bloated and needlessly complex, and the author ruins a genuinely good idea with tedious backstories and complications for the sake of complications. Christie at her best was a master of economy: the story fit together like a jigsaw puzzle, with no unnecessary pieces and no need to artificially shove pieces together. Hannah didn't meet that standard. With a better editor--- not just a copy-editor (though the dialogue could use some work), but someone to polish down the story to its deserving essentials--- she could have had a good story. As it stands, it's a decent but unspectacular one. I don't feel cheated or disappointed for it, but in retrospect I should have just read the first and last chapters.
The book is decent enough that I'll read Hannah's next Poirot novel, but I wouldn't recommend this one. If Hannah improves in future stories, I'd recommend readers just skip to the later books in the series. If she doesn't improve, I'd recommend readers avoid her novels altogether. This novel is just mediocre.