Edition: New English Library, 1975
This is one of my favourite novels, partly because I have a soft spot for the Peter Wimsey series, and partly because it is set in Oxford. It reads as though Dorothy Sayers was finally really indulging herself - she resolves the long-running romantic tension between Harriet Vane and Lord Peter, and she clearly also had very fond memories of her own time in Oxford. The plot may be self indulgent, but the writing is as good as ever; this is probably going to be the favourite among most Peter Wimsey fans.
The plot isn't actually a murder mystery, but is concerned with an investigation into a poison pen letter writer and player of nasty practical jokes in Harriet Vane's old college. The seriousness of these attacks are shown by an attempted suicide by one of the students.
It is not until Peter returns from abroad that progress can be made in solving the mystery - naturally, since he's the Sherlock Holmes to Harriet Vane's Watson; but the mystery is not so important as the relationship between the two of them and the background of Oxford in the thirties.