A mug book is a collection of photographs of criminals, typically in mug shot

Mug shot

A mug shot, mugshot or booking photograph, is a photographic portrait taken after one is arrested. The purpose of the mug shot is to allow law enforcement to have a photographic record of the arrested individual to allow for identification by victims and investigators. Most mug shots are two-part,...

s 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 then individually displaying each photo.

Mug book also has a meaning in genealogy

Genealogy

Genealogy is the study of families and the tracing of their lineages and history. Genealogists use oral traditions, historical records, genetic analysis, and other records to obtain information about a family and to demonstrate kinship and pedigrees of its members...

 and history

History

History is the discovery, collection, organization, and presentation of information about past events. History can also mean the period of time after writing was invented. Scholars who write about history are called historians...

, referring to local biographical histories published in the US in the late 19th century.