Since I am a much better reader of mysteries than a writer I feel somewhat qualified to throw my oars into the water in giving my opinion about the qualities of a good mystery. We may all have our particular settings or styles or the love of blood or lack thereof in our mysteries I think we can all unite on one thing. A good mystery requires a good story.
Maybe that is the basic element of any book in any genre, even in non-fiction. The story has to get our attention and make us want to read more. For mystery books there has to be some element of the unknown that we are promised will be revealed if only we hang around long enough. Or even if we know 'who dun it' how the perpetrators are brought to justice or not may be enough to hold us fast to our seats and keep us turning the pages.
But how the story is told and the definition of the main characters are close behind in terms of factors that make up a good mystery. Style, pace and plot development are keys to ensuring that the reader is not just entertained, but engaged along the way. The sub-genres of mystery start diverging here, particularly around style which tends to involve detailed and sometimes flowery descriptions in cozies or technically detailed forensic talk in police procedurals. But they all come back together when it comes to the flow of the story. Good mysteries in all forms have a rhythm that somehow just seems right. Great mystery writers have the 'Goldilocks' touch: not too fast, not too slow, just right!
Great characters are another key to great mysteries. We all remember the giants like Poirot or Miss Marple or Rebus or any number of great cat writers. But I find that it is actually the sub-cast of characters that separate the great from the good. And it's not usually the person or persons who get killed that are the most interesting. It's the Corporal under the Sergeant, or the old friend who always shows up with advice or a bottle of scotch at exactly the right time.
But what really sets the mystery category aside from all other writing is the added characteristic of surprise. Every mystery book has a few twists and turns but a great mystery book has an absolutely brilliant surprise. It may be that the butler didn't actually do it, but he was certainly involved in helping the less than legitimate heir bury the bodies. Or an unheard of relative who surfaced just after the will is read orů you get the picture.
Reading a great mystery book is like having a candle to light the way down a dark and unfamiliar hallway. You don't know what you are going to find down there, but you just have to go and see for yourself.