A mug book is a collection of photographs of criminals, typically in mug shots taken at the time of an arrest. A mug book is used by an eyewitness to a crime, with the assistance of law enforcement, in an effort to identify the perpetrator. Research has shown that grouped photos result in less false-positives than individually displaying each photo.
Mug book also has a meaning in genealogy and history, referring to local biographical histories published in the US in the late 19th century.
References
Mug book Wikipedia(Text) CC BY-SA