The Mysterious Affair at Styles, by Agatha Christie
We were sitting in the library. Several hectic days lay behind us. In the room below, John and Mary were together once more, while Alfred Inglethorp and Miss Howard were in custody. Now at last, I had Poirot to myself, and could relieve my still burning curiosity.
Poirot did not answer me for a moment, but at last he said: "I did not deceive you, mon ami. At most, I permitted you to deceive yourself."
"Yes, but why?"
"Well, it is difficult to explain. You see, my friend, you have a nature so honest, and a countenance so transparent, that - enfin, to conceal your feelings is impossible! If I had told you my ideas, the very first time you saw Mr. Alfred Inglethorp that astute gentleman would have - in your so expressive idiom - 'smelt a rat'! And then, bon jour to our chances of catching him!"