tl;dr: 'The Seven Deaths of Evelyn Hardcastle' is an insanely complicated, thought-provoking, genre-bending murder mystery with an incredible pay-off.
Simplistically speaking, this book takes an Agatha Christie-Esque whodunnit and blends it with plot elements from 'Groundhog Day' and 'By His Bootstraps'. Aiden Bishop, our protagonist, is charged with solving the murder of Evelyn Hardcastle. She is going to be murdered at night during a party thrown by her parents at their estate. If he is not able to do so, he will have to live the same day again but this time as a different character at the party. He remembers some elements from his previous incarnations but the extent to which he retains his memory and behaviour depends on the character he is currently playing. And to top all this, somebody is out to kill him one character at a time.
The plot is the star of the show. A book with this kind of mystery will stand out even if it weren't for the other science fiction/fantasy elements. It feels like something Agatha Christie would write (or maybe she has since she was so prolific) in terms of setting, characters and twists. The story takes place in an estate set in 1920-1930s England with sharp differences between the aristocracy and the working class with all the accompanying quirks of that era. The twists and turns as the main character uncovers what is happening is the final stamp of the Agatha Christie template.
On top of this, the Groundhog Day/By His Bootstraps elements deepen the mystery and the lack of understanding. As the main character goes through the various iterations, he discovers more and more about what is happening, where he is and why it is happening. This discovery works well as a device for the reader to figure out what is happening. The information Aiden gathers is the exact information that the reader also gets. It is more like a Poirot mystery in that sense (motivations and human behaviour) than a Holmesian one (forensic evidence and drawing profound deductions). But be warned, the plot can get overwhelming at times, especially when the handoffs and interactions between the characters start getting more frequent.
The writing deserves praise too. Stuart Turton has done a bang-up job in communicating an ambitious plot such as this clearly and concisely to the reader. I like how he has structured the book such that each iteration of Aiden has a chapter to itself. The structure helps a lot towards making sense of the whole plot. The pacing is mostly decent but some parts in the middle, drag a little bit.
Overall, I highly recommend 'The Seven Deaths of Evelyn Hardcastle'. It is an excellent, intellectual, challenging one's mind kind of book. The Goodreads reviews fall into two buckets - either people love this book or have given up on it. The common factor is whether they enjoyed the complicated plot or not. The reader needs to put in the effort to get the maximum enjoyment from the story - else pass on this.