This is one of Christie's better efforts in terms of characterisation and plot development. As with all studies of a crime that occurred long ago, pace is necessarily sacrificed in the pursuit of information. Christie compensates for this by giving a very limited number of possibilities and introducing a lot of highly entertaining dialogue into the gradual development of the solution. Because of these factors, the rules of the puzzle are not typical Christie. Gone are the hundreds of suspects in an obvious case of murder, allowing you to play the guessing game if you are too idle to try and work it out. With this novel, you have to work out if there has been a crime and then, I suggest, use the slow pace to try and work out every aspect of the solution. Guessing is easy in this case, but will only give you part of the puzzle - even if you guess right - so what's the point? The two clues that Poirot bangs on about are all you need to work out everything by about three-quarters of the way through the book. Ellery Queen would have put in a challenge to the reader there. Then you can have the very great satisfaction of smugly nodding your way, in great detail, through Poirot's explanation.
WARNING Possible spoiler: Readers under 40 might find the behaviour of one of the characters, as described in the solution, utterly unbelievable. I would ask them to think that when the incident occurred, people were very different - more sure of what was right and wrong and, therefore, more able to decide and do what was "honourable". I hasten to add that this does not make them "better" than more recent generations, they just had a more simple outlook that made these decisions easier. Personally, I think a little uncertainty is a good thing; it makes you less likely to make mistakes!