Murder on the orient express is one of the best selling Agatha Christie books. It is the best detective novel in my opinion.
The novel is so good that a movie was made on it in 1974 and there is going to be a remake soon. It features a Belgian detective Hercule Poirot who solves the murder of a traveler on the Orient Express. It keeps you guessing till the end. The novel is very interesting and once you read it, you will want to read more of Agatha Christie books.