Thumbprint (Wachtmeister Studer) (1935) by Friedrich Glauser ; translated from the German by Mike Mitchell.
From the Bitter Lemon site: "Friedrich Glauser was born in Vienna in 1896. Often referred to as the Swiss Simenon, he died aged forty-two a few days before he was due to be married. Diagnosed a schizophrenic, addicted to morphine and opium, he spent much of his life in psychiatric wards, insane asylums and, when he was arrested for forging prescriptions in prison. He also spent two years with the Foreign Legion in North Africa, after which he worked as a coal-miner and a hospital orderly. In 1939, a year after Glauser’s death, the film of 'Thumbprint', the first Sergeant Studer mystery, was greeted with critical acclaim and commercial success. Studer became more famous than his creator, the mark of true success for a fictional detective."