The Saint Plays with Fire — Leslie Charteris

When the Saint and Patricia spot a country house on fire, they rush to help but are too late for one man, who is trapped inside. The dead man had been locked in a room, so Simon concludes there's a murder to answer for, despite the coroner's differing opinion. When the Saint investigates further, he gets caught up with generals, financiers, and an assassination plot designed to start a war.