Smoke and Ashes by Abir Mukherjee

India, 1921. Haunted by his memories of World War I, Captain Sam Wyndham is battling a serious addiction to opium that he must keep secret from his superiors in the Calcutta police force.

When Sam is summoned to investigate a grisly murder, he is stunned at the sight of the body: he’s seen this before. Last night, in a drug-addled haze, he stumbled across a corpse with the same ritualistic injuries. It seems like there’s a deranged killer on the loose. Unfortunately for Sam, the corpse was in an opium den - and revealing his presence there could cost him his career.

With the aid of his quick-witted Indian Sergeant, Surrender-Not Banerjee, Sam must try to solve the two murders, all the while keeping his personal demons secret, before somebody else turns up dead.