This is one of my three favorite Lord Peter Wimsey novels (the other two are Clouds of Witness and The Unpleasantness at the Bellona Club), and it's my favorite of the "later period" (1930s) stories (in some part because it doesn't feature Wimsey's paramour, Harriet Vane, whom I often found rather dull). This book is Wimsey at his most whimsical, though because it is to some degree an extreme example of Wimsey's character, it's probably best enjoyed by people who have read the earlier books.
Sayers apparently worked in the advertising business herself for some years, and in this story Wimsey goes undercover as "Death Bredon" (his middle names) at Pym's Publicity to investigate the death of a copy-writer who fell down a spiral staircase. As a result, Sayers pokes all kinds of fun at the advertising business, as well as drawing an enlightening sketch of what that business is like. More than one person who's read this novel has commented to me that it seems that advertising hasn't changed much in the last seventy years!
The victim himself had been running with a fast, drug-taking crowd, which Wimsey infiltrates to tragicomic effect, and when his contacts with this ne'er-do-well group meet his upper-class family later on, he's put in the surreal position of... well, read the novel; the ultimate payoff of this thread is one of the funniest moments in the whole series! The book also includes a chapter featuring everyone's favorite incomprehensible English sport: A Cricket match, which as it turns out fits right in with the rest of the book in both style and outcome.
The mystery itself is about average for Wimsey's adventures, and is a bit more hard-core than we'd usually expect. But that aside, this is a funny, flamboyant, and educational novel, perhaps the most rewarding overall of all of Lord Peter's stories.