Black Mass

"Black Mass" is a book based on real events, telling about the unsavory side of American life, and it is one of the best investigations into crime in the USA throughout the 20th century.
The main "hero" is James "Whitey" Bulger,... a descendant of Irish immigrants, a ruthless, uncompromising criminal from South Boston. He served in the US Air Force, went to prison, and upon his release, immediately joined the Boston Winter Hill gang and rose to become its leader. According to police, Bulger is involved in 19 murders and engaged in all kinds of criminal activity.
A rising star of the FBI, and also Bulger's childhood friend, made the mobster a lucrative offer - protection in exchange for information about the organized crime group.  Connolly did not suspect that he was making a deal with the devil, which would ruin his life. 
All the characters are alive, except for Whitey Bulger.  The plot of the book is the basis for the film of the same name starring Johnny Depp in the lead role.  In another adaptation, "The Departed," Bulger was played by Jack Nicholson, while the FBI agent Connolly was portrayed by Matt Damon.