the secret of chimneys by agathat christie

A young drifter finds more than he bargained for when he agrees to deliver a parcel to an English country house. Little did Anthony Cade suspect that a simple errand on behalf of a friend would make him the centrepiece of a murderous international conspiracy. Someone would stop at nothing to prevent the monarchy being restored in faraway Herzoslovakia.
The combined forces of Scotland Yard and the French Surete can do no better than go in circles – until the final murder at Chimneys, the great country estate that yields up an amazing secret.