It may come to you as something of a shock after all these years defending and promoting the traditional mystery but I’m not overly fond of whodunits. I like them for sure, but they are not my favourite brand of crime fiction.
Maybe you’ll have a faster recovery if I tell you what I mean by « whodunits ». I mean this kind of mysteries that hinges exclusively on finding who did it, no other question of substance being asked. They can be quite fine stories or books otherwise but most of the time they don’t make a smash with me. The problem is entirely mine: such books aim at surprise and it’s hard being surprised when like me you have acquired the bad habit to suspect everyone. Not that I’m never fooled, for it happened precisely this year with Carr’s The Plague Court Murders, but 1°) It’s Carr; 2°) The book has plenty more to offer besides that.
Other than its predictable nature once one has mastered the rules and the tricks, the problem with the whodunit is that most of the time it isn’t very suspenseful since very little happens and the outcome is never in question. What few « action » there is is mostly the detective interrogating people, which is fine to convey information and clues but somehow tedious as a way to build dramatic tension. This is what I called « straight plotting » and the only way it can avoid boring the reader is by adding subplots, often involving the private lives of the detectives since they are the only characters one can get inside their minds without giving too much away. This is how we end with the contemporary mystery novel in which the detective is as important as the case they investigate.
This portrayal of the genre may sound grim but it is only because it is so hard to master. The writer needs to be at the same time a clever plotter (so as to surprise the reader) a good storyteller (so as to maintain interest throughout) and a gifted « traditional » writer (so as to not depend exclusively on the plot) Most whodunit writers manage at best to fill in only two requirements – Ngaio Marsh comes to mind – and only one managed to score them all at her peak. Howard Haycraft famously warned up-and-coming writers to avoid the impossible crime trope because of its exceedingly difficult nature; he might have added the « pure » whodunit as well.
This doesn’t mean that the « Who? » doesn’t matter, or no longer does and would no longer be an issue as a famous reviewer put it. What this means on the other hand is that it is shaky ground to build a mystery exclusively upon. If you reallu want to write a crime novel that is only about finding who killed Lady X, that’s fine, but you’d better have an hell of an ace up your sleeve or massive writing chops. This is part of the challenge and consequently of the fun, but be warned the rate of failure is distressingly high.