The first case of Hercule Poirot. He is a Belgian just immigrated to England. Nothing about him makes him look like one of the greatest detectives ever. The woman who helped him settle in his new country was murdered, Poirot is forced to use his "grey cells" for the first time (but definitely not for the last time).
Agatha Christie wrote this novel as a bet that she can write a mystery novel with all the clues given to readers, which still will be really hard to figure out before the book (Poirot in this case) explains it all. Not as strong as her later books, it is still belong to classic mystery section.