This is the first book Agatha Christie published and the one that introduced Hercule Poirot to the world. The story was inspired by her experiences working at the Torbay Dispensary during WWI and of all the criticisms that could be thrown at her stories, no one could accuse her of not knowing her poisons. This is especially true of this book where an understanding of how two chemical compounds interact is key to solving the murder. But this book uses Christie's key device of misdirection, as well as the assembling of all the suspects for the denouement, are there. What is interesting is that Poirot's moustache, which in later novels almost becomes a character in its own right, is not described as luxuriant, but as being 'very stiff and military' and isn't referred to again.
Harper Collins have been issuing facsimiles of the first editions which are wonderful. The back of the dust cover has given me hours of pleasure, did you know that in 1920 a book cost between 7s 6d net and 10s net?