Murder Must Advertise by Dorothy L Sayers is an excellent dissection of an ad agency in the 1930s. I read it first in my 20s when I was reading my way through the Golden Age authors of the 1930s. Re-reading it again I was interested to find the criticisms of advertising even more relevant today than they were when the book was first published. The comment that the rich don't pay attention to advertising because they buy what they want when they want; but the poor pay attention to adverts and stay poor because they feel they have to buy all the things advertised to improve their lives is definitely applicable to the 21st century.
The mystery and the murder - which has happened before Wimsey comes on the scene - are complex involving illegal drugs and the fashionable set who use them - and very much relevant today. I enjoyed this book because it brings in Peter's brother in law - Charles Parker, theology reading Scotland Yard detective. The insight into advertising is fascinating especially the way the exact right words for adverts are decided upon. What is right for one company will be rejected by another. Well worth reading even today.