This was the 5th Hercule Poirot mystery but is quite a bit different from MYSTERIOUS AFFAIR AT STYLES or many of the more famous later ones like MURDER ON THE ORIENT EXPRESS or DEATH ON THE NILE. The plot is more fantastic, involving a vast international conspiracy of supercriminals and has Poirot taking a much more active role than usual.
The story opens with Hastings planning to surprise Poirot by arriving from South America unannounced only to be surprised himself by Poirot's own imminient departure - for South America. A filthy stranger bursts into Poirot's apartment with a cryptic message which sets the two detectives on a long and twisted trail to save the world.
This book was published in 1927 not long after Christie's first marriage ended (and her mysterious disappearance). She was in deep financial trouble and took a suggestion to link together a series of previously published stories. (Information from Agatha Christie's autobiograhy).
This resulting book was immensely popular when it was first released but has not aged that well, in my opinion. If you are, like me, an advid Christie fan, you will probably find this book amusing but if you are new to Christie's work or are only familiar with the later books you probably would be better served with another choice.