Eighty years ago, S.S. Van Dine - a pseudonym for Willard Huntingon Wright and the author, most notably, of the Philo Vance detective novels - came up with a list of twenty rules for how detective fiction should and should not be written. I'd invoked these rules at one of my panels at the LA Times Festival of Books and figured it would be fun to revisit them. Obviously, all of them have been broken in the 80 years since - sometimes well, often not so well - but #15, I think, still matters the most:
The truth of the problem must at all times be apparent — provided the reader is shrewd enough to see it. By this I mean that if the reader, after learning the explanation for the crime, should reread the book, he would see that the solution had, in a sense, been staring him in the face-that all the clues really pointed to the culprit — and that, if he had been as clever as the detective, he could have solved the mystery himself without going on to the final chapter. That the clever reader does often thus solve the problem goes without saying.
Of course, that's if your primary motive is to keep the reader on his or her toes. Van Dine thought of detective fiction as an intellectual game; what's transpired in the eight decades since is how said novels have become more about the emotional and the visceral. Or to spell it out more clearly, empathy in classical detective fiction was an afterthought; now it's a crucial component. I think that's rather a good thing.