This is definitely one of Christies best novels, right up there with Death Comes as the End, And Then There Were None, and Murder in Mesopotamia.
Yeah, ok, she might not be a literary genius, but she definitely has genius of a kind. She practically invented this type of detective novel (i'm not counting Conan Doyle. Why? Because i hate his ludicrous novels), using the brain, whilst at the same time creating great character sketches.
She doesn't develop her characters loads, but then, that is NOT what her novels are about. they're about enjoying a great puzzle. And boy, can she construct a great puzzle! The one here is probably the bestm in that is lacks Hercule Poirot for about the first three quarters (some of her best books were those which were without Poirot or Marple). through that she was able to create a good psychological detective story, then bring Poirot in at the end to tie it all up neatly and provide the solution.
The climax is great, and probably one of her best. I read this when i was about 12, and was not at all versed in the workins of a detective novel, in that it is the least likely person who did it. I just enjoyed the read, and was gladly surprised when i found out the identity of the culprit. No doubt, those with mroe experience would have been able to spot the killer straight away. However, this remains one of her best novels, and one which i will enjoy reading again and again.