Perhaps to Kill — Howard Charles Davis

Ostensibly, Mr. and Mrs. Rudolph Nabb are cruising the Dodecanese Islands. However, in their yacht is a Mannlicher rifle, and on the volcanic Island of Hexiemos, living incognito, is the ruler of a Balkan State, Zeus Leru, whom Nabb and his companion have been hired to kill. When the island's Mount Karna erupts, the perils of earthquakes and lava streams are added to those of the hunt, and Nabb plays a role he had never anticipated.