Murder Must Advertise

Murder Must Advertise is the title of a Lord Peter Wimsey mystery[?] novel by Dorothy L. Sayers, published in 1933. Most of the action takes place in an advertising agency, a setting with which Sayers was very familiar. It is often considered to be the best book of the Wimsey series. It is also the title of the BBC made-for-television film version of the novel, starring Ian Carmichael as Lord Peter.

Synopsis: (Warning: Wikipedia contains spoilers.)

Lord Peter Wimsey, under the pseudonym of "Death Bredon," takes up employment as a copywriter for an advertising agency in order to discover more about the recent mysterious death of one of the employees. In the process he discovers much of the convoluted private lives of the other employees, not to mention getting a feel of what it is to actually work for a living. Eventually he traces the connection to a drug-smuggling operation, which he proceeds to infiltrate and uncover. Wimsey makes the connection between the drug-smugglers and the employee who has become their tool, and thus has been motivated to murder.