There is a problem to be solved in mystery stories - sometimes these are solved by police, detectives, spies and, in lots of stories, children and young adults.
As well as these suggestions and recommendations, you can also look at the library catalogue here, where you can do a subject search, using the term 'mystery stories'.
A crime of fashion by 
The mystery of the man with the black beard by 
Dr. Critchlore's School for Minions by 
Phyllis Wong and the Forgotten Secrets of Mr Okyto by 
