What constitutes a mystery? Most people would agree it is a story that keeps you in constant suspense with characters who are experts in figuring out such obscure details that it makes you have the "why didn't I think of that moment". Certain mystery novels with have a character that is sometimes filled with such useless knowledge it becomes useful! Also the story must have lots of twits and turns and even a few predictable moments.
In Chandler's essay he notes that "genres can therefore be seen as "fuzzy" categories which cannot be defined by necessary and sufficient conditions." The Alienist by Caleb Carr is one of those books! It can be labeled as mystery but also have many sub-genres. The cover of the book itself leads the reader to understand that this is going to be a very dark and mysterious novel. After three short chapters you are pulled into the story with the description of a very grotesque crime scene. The "detective" team each contribute their own talents and specialties that help solve the murders. As the story carries on you find out more and more clues that lead you in twists and turns leaving the reader in constant suspense; filled with guesses of how to solve the murders.
The story keeps the reader on the edge of their seat and that's what a mystery should do, same goes for movies.I don't want to categorize this book as a "typical" or a "generic fiction" as Chandler states. I feel if one is writing a mystery there can't be much deviation from the "rules of a good mystery. It goes with the old idiom of if it ain't broke don't fix it!"
Although if you look at mystery novels from the past to present they seem to be a bit more graphic in the present day. My guess as to why this is, can be due to our generations movies and books becoming more graphic to keep people interested and possibly to pay attention. Also as our technology advances so do our ways to solve the crime. One can see more forensic science in present stories as compared to Caleb Carr's story which takes place in the mid 1800's. For example there are a pair of scientists in The Alienist that suggest a person's eye can take a mental picture of the last thing they saw, needless to say they fail to see any pictures when they examine a person's eye. If this was present day or even future technology that was being written about they would see the whole murder!