Agatha Christie 
Double Sin 

When Hercule Poirot takes a holiday to Ebermouth with his friend Hastings, he didn’t expect it would turn into a case. But when a young woman named Mary Durrant asks Poirot for help after miniatures that were mailed to a client go missing, the great detective springs into action.

Were the miniatures simply misplaced or deliberately stolen? And if stolen, by whom? Poirot must rely on his 'little grey cells’ to unravel the mystery and identify the true culprit.