A well - written mystery novel often has a complex plot. There are many layers to the mystery that keep the reader guessing. For example, in 'And Then There Were None', the mystery of who the killer is among the group on the island is full of twists. Also, strong characters are important. A detective like Sherlock Holmes in 'A Study in Scarlet' has unique traits that draw readers in.
For a mystery novel to be well - written, it should have an unpredictable ending. 'Gone Girl' is a great example of this. The twists at the end completely change the reader's perception of the story. The writing style also matters. A vivid and engaging writing style like that in 'The Big Sleep' can bring the mystery to life. And the motives of the characters involved in the mystery need to be believable. If the motives seem far - fetched, it can ruin the overall mystery.
One key factor is suspense. In a well - written mystery novel such as 'The Girl with the Dragon Tattoo', the author builds suspense throughout the story. The reader is constantly on edge, wondering what will happen next. Another aspect is the setting. A spooky or isolated setting like in 'The Hound of the Baskervilles' can add to the overall mystery. And of course, the clues need to be cleverly placed. They should be there for the reader to pick up on, but not so obvious that the mystery is easily solved.