Warning: Wikipedia contains spoilers.
In this book detective Hercule Poirot is travelling on the Orient Express. On the journey, Poirot meets a very close friend Bouc, who works for the Compagnie Internationale des Wagons Lits. When American millionaire Samuel Edward Ratchett is mysteriously stabbed to death, Poirot and Bouc work together to solve the case. Key to the case is Ratchett's involvement in the Armstrong tragedy, in which a baby was kidnapped and then murdered. (The fictitious Armstrong case was apparently inspired by the real-life kidnapping of the child of Charles Lindbergh.)
The twelve suspects include Hector MacQueen, Hildegarde Schmidt, Count and Countess Andrenyi, Ms. Hubbard, Greta Ohlsson, and Colonel Arbuthnot.
The book was made into a 1974 movie (starring Albert Finney as Poirot, Lauren Bacall as Mrs Hubbard, Ingrid Bergman as Greta, Jacqueline Bisset as Countess Andrenyi, Sean Connery as Colonel Arbuthnot, John Gielgud as Beddoes, Wendy Hiller as Princess Dragomiroff, Anthony Perkins as MacQueen and Vanessa Redgrave as Mary Debenham) as well as a made-for-television movie in 2001.
Warning: Wikipedia contains spoilers. This book was also noted for its surprise ending, where it is revealed that all of them did it.