Mystery fiction is a loosely-defined term. 1.It is often used as a synonym for detective fiction or crime fiction in other words a novel or short story in which a detective (either professional or amateur) investigates and solves a crime mystery. Sometimes mystery books are nonfiction. The term 'mystery fiction' may sometimes be limited to the subset of detective stories in which the emphasis is on the puzzle/suspense element and its logical solution (cf. whodunit), as a contrast to hardboiled detective stories, which focus on action and gritty realism. 2.Although normally associated with the crime genre, the term 'mystery fiction' may in certain situations refer to a completely different genre, where the focus is on supernatural or thriller mystery (the solution doesn't have to be logical, and even no crime is involved).