Gosick

Kujo Kazuya is studying abroad in Sovuria (a fictional country in Europe), where he meets the mysterious Victoria. Possessing a brilliant mind, which she calls "fountain of wisdom", she seems to know everything, although she never attends classes and is restricted to her room and the schools library. (Source: taken from the light novel summary) Note: Volume 2 contains the extra "Suigyo no Majiwari" (水魚の交わり).

1931, New York. Victorique and Kazuya now live new lives as private detective and newspaper reporter. Victorique is asked to handle a case regarding a string of murders related to gangs, while Kazuya heads to a news coverage of a well-known psychiatrist famous for "curing the psychological traumas of patients by using the science of the heart." The mystery of the serial murder case lies in the fact that the scenes of the crime, the methods of killing used and the profiles of the criminals vastly differ from one another, while the psychiatrist utilizes the mysterious dream diagnosis for counseling. Before long, a link forms between the two cases, transforming into a dreadful conspiracy... (Source: Kadokawa, translated)

Set in a fictional European country in 1924, a Japanese exchange student meets a mysterious, brilliant girl who only leaves the library to sleep. Her brother, a detective, relies on her mind to solve difficult mysteries, several of which draw her away from the library. With a drop of sweet romance, you will immediately become addicted to Gosick! *Note: GosickS is a series of side stories taking place before, after, and in between several of the cases covered in Gosick.*

Side story from Gosick.