This article by Howard Kohn, the Pulitzer prize winning editor of Rolling Stone magazine, goes into great detail about the mob's connection to the JFK assassination and the HSCA investigation.
It was written prior to Carlos Marcello's confession to an FBI informant, but even then, the mafia's role in the case was quite obvious. Robert Blakey, the head of the HSCA, wrote a book on the case, stating his belief that Marcello was behind the assassination - also before Marcello confessed.
http://jfkhistory.com/mobsters/Nicoletti.html