If you don't already know it, this is Poirot's last case.
I came across this book after I had read MANY other Poirot's cases and I was familiar with the surroundings. Lady Agatha takes us back to Styles Court, where we first met the little Belgian man with the egg shaped head. And so, the circle is closed.
This is, in my opinion, the most mature of Christie's stories. Poirot faces the ultimate assassin: an individual capable of the perfect crime. He understands his methods, but also finds it impossible to intervene. And so he takes the matter on to his own hands, although knowing that no crime can go unpunished.
Maybe the essence of the book is in the last few lines, hidden by a mark on a man's forehead. Don't miss it.