A more thorough explanation:

Definition: A mug book is a collection of photographs of criminal suspects that is maintained by law enforcement agencies such as the FBI and police departments. These photographs are used to identify criminal offenders.

Example: When a crime is committed, law enforcement officers may use a mug book to try to identify the suspect. They will look through the book to find a photograph that matches the description of the suspect. If they find a match, they can use the photograph to help them track down the suspect.

Another example: A police department may keep a mug book of known gang members in their area. This can help them identify suspects in gang-related crimes and track down members of the gang.

These examples illustrate how mug books are used by law enforcement agencies to identify criminal suspects and track down offenders.