This one is hard to rate. In terms of deception, it's up there among the cleverest of her novels. I had actually just read A Talent to Deceive which goes through the devices she uses, so I knew there was something strange - as in Ackroyd-strange, genre-rule-bending - about who the murderer was, and yet I was completely fooled - as always.
And the epistemological point about the little things that are unnecessary and therefore become meaningful once the narrative is seen from the right angle, is both ingenious and ultimately the reason why Poirot is the greatest crime fiction character of all times.
But the flaws... The psychologising mumbo jumbo in the final Poirot speech; the over-abundance of false identities; and worst of all: the implausibility of (a) the extremely complicated setup at the murder scene, and (b) Poirot's very detailed knowledge of the exact details, which he could not have known or deduced, at least not from what information we are given in the book.
So: at the same time one of her strongest and weakest efforts. Hence the three stars.