eathFiction by Agatha Christie.
This is one of Agatha Christie's best detective stories, and definitely my favorite of the Hercule Poirot books. I've read it before, and recently enough to actually remember whodunnit, but that knowledge actually enhanced this reading, oddly enough. I enjoyed looking for the clues leading to the conclusion.
That being said, if you haven't read this one, DO NOT READ SPOILERS. The surprise is the best that way. Obviously.
Incidentally, I hadn't realized that this book was so early in Christie's writings; it seems she tried to retire M. Poirot rather quickly but instead ended up keeping his character in her repertoire until her death.
I also read recently by this author: The Secret Of Chimneys