Good twist and the whole mystery made sense, except I was suspicious of the murderer from the beginning and how they were brought into the whole story.  Additionally, the doctor writing in 1st person for the whole novel was a bit of an issue for me bc it made the ending a bit idk unpredictable to the point of being lazy??  
I did enjoy it, but it wasn’t my favourite old timey mystery, and oh my god - Caroline was literally my favourite character wth I wish she was the one employed by Poirot.