Murder on the Orient Express - by Agatha Christie

Someone had died in his box, but it seemed to be a perfect chamber of secrets, which meant "no crime possible". As Poirot investigates further, the case seems more complicated and Poirot comes up with two very different conclusions. Agatha's novels always take a huge turn when I find out who the killer is, demolish the theories I've established before, and then slowly reveal the truth behind the events that you never thought of.