I think Christie delighted herself a bit to much in the idea for this book. To start with, the conversations overheard at the beginning of the story are so over the top and disjointed you wonder if the author began writing with any idea of how to get where she was headed.
When the reader is eventually let into the story it is without any of the artistic flare Christie readers will admire in her other works. The clues are open and obvious, and the story line follows more "How to solve a mystery." than "What is the conclusion to this puzzle?"
To finish it off the conclusion is most unsatisfactory, much like this one