Sad Cypress is one of Christie's books which stand out in my memory, and that now and then I fancy reading again. The twist at the end, which is surprising and quite prosaic, the magic of the Christie atmosphere, in between style and tension is there, and the implied importance of heredity and class are almost Victorian!
But the most interesting feature as far as I'm concerned is in the character of Elinore Carlisle. Her skilfully repressed passion and devotion echo a side of the British character that often goes untold, and very possibly hints to the passionate side of Agatha Christie herself, who wrote romantic novels under the name of Mary Westmacott, and loved her first husband Archie Christie so intensely as to actually lose her mind temporarily when he left her for another woman (she experienced a brief "fugue" where she lost her memory and signed into a hotel with the name of her husband's new flame).
As for plot, narration does feel somewhat disjointed as the story is narrated in retrospective, and in parts through letters, but it really does work, and the ending doesn't disappoint.
Poirot joins the story quite late, which makes for a change.
Elinor Carlisle remains one of my favourite Christie women.