I am re-reading this book. It's quite a strange book and is a completely different style to any other of Agatha's books. I believe she first wrote a series of short stories which were published in papers.
She was going through a fairly traumatic time in her life and the publishers were pressing for a book. Her brother in law suggested that she string the short stories together to form a full length book. The result? The Big Four.
This reminds me very much of Conan Doyle Poirot and Hastings could easily be Holmes and Watson.
I would be interested in what anyone else thinks?