It seems to me that there are two kinds of mystery novel: the Donald Sobol model and the Dick Francis model.
Donald Sobol is the author of the Encyclopedia Brown series. These are short stories for young adults, and in each one the eponymous Brown solves a crime. The reader is presented with all of the evidence needed to solve it herself. At the end there is usually something on the order of \"How did Encyclopedia Brown know that Old Man Jenkins was lying?\" and the answer is printed upside down on the page. It\'s usually something that was mentioned earlier in the story, something that a bright ten year old could notice, like, \"If Old Man Jenkins was at the bar during the robbery, how did he know about the secret basketball game?!\"
This type of mystery emphasizes the mechanics of the crime, and builds up to the \"Aha!\" moment at the end, when the detective reveals the solution.
Agatha Christie is one of the better examples of this, in my opinion. She writes (a few) fascinating characters and provides (some) neat social commentary / period writing, but the real thrill is in realizing that the clever twist was potentially there for anyone to read. That even if you didn\'t solve the murder on the Orient Express, you could have all along. I also dearly love Dorothy Sayers, and feel that she belongs in this category as well. Her obsession with railway timetables is a bit much for me, however.
The second type (or the point on the other end of the continuum, if you prefer) is Dick Francis. I read a great many of his novels in my youth (enjoying them all, I should add!) and they all follow a strict formula: The main character is introduced*. He is bluff and manly without being too macho about it. Some connection to horse racing is established, and then a crime is committed. The perpetrator of the crime is rarely mysterious, and in several novels we\'re even told up front who did it and how. The main character pursues clues, falls in love, and is kidnapped. Then he\'s tortured for a while (either by the villain directly, or because the villain has chained him up somewhere uncomfortable for a long time). Eventually he\'s rescued, sets a trap, and sees the villain arrested.
Someone told me once that modern mysteries were less about crimes per se, and more about the psychological aspects of the characters who commit--and investigate--the crime. This certainly rings true for the Dick Francis novels that I\'ve read--there\'s never any crazy plot twist at the end, never a revelation sprung on the reader at the eleventh hour. But it\'s terribly interesting to watch the hero figure it all out, and to learn why the villain behaved so poorly.
Arthur Conan Doyle fits into this category too, although not as perfectly. Sometimes the resolution of Holmes\' case is a surprise to the reader, and we\'re never told whodunnit right at the front the way Dick Francis would have, but the main point of the exercise is to watch Holmes work. Poirot and Whimsey both solve mysteries, whereas Holmes has adventures, and his key clue is usually something terribly obscure about pipe tobacco.
I\'ll mention one more example: I am currently reading Rex Stout\'s series of Nero Wolfe novels. These are basically in the Christie / Sobol mold, but they\'re not a perfect fit. The identity of the killer is always revealed at the very end of the novel (like Christie), and there\'s always a cunning twist that provokes an epiphanic reaction, however I don\'t think that it\'s fair to say that the reader possesses all of the clues necessary to solve it on their own.
However, one thing that I love about the series is that Stout is incredibly good at making me feel as if I had all the clues I needed. At one point the point of view character, Wolfe\'s assistant Archie Goodwin, says that working for Wolfe
was like being with him in a dark room which neither of you has ever seen before, as he describes all of its contents to you, and then when the light is turned on his explanation of how he did seems sensible because you see everything before you just as he described it.Of course the author has created the room and all of its contents himself, so it\'s easy for his detective to describe them in the dark. But the feeling is pervasive. Wolfe declares (usually around page 80) that of course he knows who did it, but just needs to get one or two bits of proof before revealing it. And then (in the final chapter) he describes the full details to a room full of suspects. It is very satisfying!
I also love it because Wolfe is brilliant, and courteous, and terribly eloquent. And he\'s also a recluse, afraid to leave his house; he\'s immensely overweight; always (well, almost always) in it for the money; and has a beautiful chemistry with the narrator, Goodwin. He has a fearsome reputation as a world-famous detective, but he is equally respected (in different circles) for his skill at growing orchids.
*One queer pecularity that I love most is that this main character almost always has a different profession. One novel will star a photographer, another a painter, a third a poet. A couple have been jockeys, and one was a toymaker. All professions are very well researched, and interesting to read about--often more so than the actual crime!