Outstanding. Perhaps Christie's best. The whodunit is technically proficient, but where this book really stands out is in the psychology. When it's down to three, and when it's down to two, who's going to do what? It's Hitchcock-type suspense, in a book. I literally read it in less than 24 hours. 
 I'd read this a couple of decades ago and rated it highly, but rereading it now in my 40s, I was struck by how truly epic it is. I can not recommend it more highly. Stop what you're doing, go read it now. If you've seen any of the three movies based on the play adaptation, fuggedaboutit. Read the original.