(reread) 
 A deserved classic. 
 It's interesting how many of Poirot and Hastings' notable characteristics are there from the off. Poirot's backhanded compliments, and Hastings' obliviousness to them, are a particular delight. 
 The plot is reasonably classic Christie, full of little misdirections and barely spottable clues. It has been long enough since I read this that I had forgotten whodunnit, so the puzzle aspect was there in full joy for me. I fell for trap #2, and thought for a long time it was actually (view spoiler)[Cynthia, despite her obviously having been drugged (reasoning that this could have been self-administered, or, as it turned out to actually be, nothing to do with the murder) (hide spoiler)]. The only slight difference between this and most Christie is that there's only one murder (not an increasingly desperate murderer making it to 3 or 4), and that there don't appear to be any same sex couples (seriously, the number of "lady companions" & "old friends living together for company" in Christie is a joy). 
 Similarly, Christie's writing style is pretty fully-formed here. For a first novel, that's actually astonishing. Oh and while I'm here, I have no truck with those who say that because Christie wrote lots of page-turners in easy-to-understand language that means she's not a great writer; to fit that many ideas and that much creativity and that much sly wit into simple texts is a lot harder than doing the literary equivalent of Yngwie Malmsteen-esque wankery, and more people should respect that.