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The General's Envoy by Anthony Conway, 2nd in his Caspasian series of early 20th century military action adventure thrillers, this installment originally out from Hodder & Stoughton in 2001.
Warlord China, 1927. Seconded to China by a vengeful Indian Army high command, Gurkha Captain John Caspasian finds Shanghai a cesspit of despair and corruption as refugees pour into the violent, disease-ridden city. He escapes on a solo mission to contact a general whom the British see as a bulwark against the revolutionary leader Chiang Kai-shek.
Once he has left the relative safety of the International Settlement for a gunboat on the Yangtze though, Caspasian smells danger. The supposedly friendly General Mok turns out to be a bloodthirsty sadist whose armoured train has struck terror into the Chinese countryside.
And Caspasian has made two implacable enemies. One is a Chinese criminal who first clashed with him over a rescued bar girl. The other is a former British officer who once saved Caspasian's life...