Doubled in Diamonds

Doubled in Diamonds is a 1966 spy thriller novel by the British Victor Canning. It is the second in a series of four novels about Rex Carver, a private detective drawn back into his old profession of espionage.

Synopsis
Carver is hired to find a man who has failed to claim an inheritance, who is turns out has been involved in a diamond heist. His mission takes him to Dublin and France, and draws him into a plot overseen by two Chinese twins, who are agents of the communist government, to sell drugs and earn hard currency for their country.