The principle difficulty with mystery fiction is the business of telling two stories at once. There is the leading narrative, which must be lucid as any story, composed of incidents and characters that are intelligible and reasonable to the reader, and then there is the covert narrative, built out of clues, hints and sheer misdirection that keeps pace with the primary narrative but must ultimately be unravelled and accounted for as clearly and logically as its counterpart.
Mark Haddon's The Curious Incident of the Dog in the Night-Time cleverly plays with the conventions of mystery by shifting this difficulty from the mystery (the murder of the eponymous dog) to the character of the detective. What is most engaging and moving about the novel is that it forces the reader to read its narrative as though it were a case and points to the distinction between the bare facts and meaning on which detection itself depends.
To explain: Haddon's protagonist and nominal detective, Christopher, is autistic. The condition is never named, but Christopher cannot tolerate lying, cannot read faces or parse the complex emotions that expressions communicate, and cannot tolerate figurative language, although he is an extraordinarily precise observer of facts and details, with a mathematically acute mind. Christopher's revulsion at lies is perhaps aligned with his desperate need to maintain distinction, order and unambiguity in the face the overwhelming stream of reality. His memory for facts and details makes him, in the Holmesian mode, a perfect observer, potentially a perfect detective. All this we understand by inference and reading.
But although Christopher discovers the truth about the mystery, and thereby the suppressed truth about his own family, the solution is trivial compared to the complexity of adult emotions, needs and betrayals to which Christopher is almost completely blind. As a detective, Christopher can uncover the means, but the motives are forever obscure for him. As the last pages in the book outline Christopher's mathematical proof, ending with the triumphant QED, we realise that some proofs and some mysteries will remain outside of Christopher's perceptions.
The reader's task, then, in this work, is not to puzzle out the solution but to experience Christopher's puzzlement, and to understand that though plots are resolved by logic, stories are only brought to life by imagination and sympathy.