A Rising Man by Abir Mukherjee

The winner of the Harvill Secker/Daily Telegraph crime writing competition.
Captain Sam Wyndham, former Scotland Yard detective, is a new arrival to Calcutta. Desperately seeking a fresh start after his experiences during the Great War, Wyndham has been recruited to head up a new post in the police force. But with barely a moment to acclimatise to his new life or to deal with the ghosts which still haunt him, Wyndham is caught up in a murder investigation that will take him into the dark underbelly of the British Raj.
A senior official has been murdered, and a note left in his mouth warns the British to quit India - or else. With rising political dissent and the stability of the Raj under threat, Wyndham and his two new colleagues - arrogant Inspector Digby and British-educated but Indian-born Sergeant Banerjee, one of the few Indians to be recruited into the new CID - embark on an investigation that will take them from the luxurious parlours of wealthy British traders to the seedy opium dens of the city. The start of an atmospheric and enticing new historical crime series.