There is a big difference between a book where a character solves a mystery and a story where the reader can investigate too. In the second kind, clues appear before the ending, theories can be compared, and the solution rewards attention.
What makes a mystery fair
The key facts should be available before the final explanation. A good story can lead the reader down a false trail, but it should not hide the decisive information until the last page.
The reader gains a personal goal: to discover whether their own theory was right.
