There is a sixth family mentioned in the book, name and all. They were at one time the most ruthless family in Sicily, but they emigrated to America. The name has slipped my mind, but they were quickly removed from power becuase of their stupidity and barbaric nature. I know that the Corleones used them to get Mike back to America, by convincing the boss to make one of his sons who was in prison confess to the murders of Sollozzo and McKluskey. Also, this family offers up hostages in times of trouble, i.e. when Michael was meeting with Sollozzo and McKluskey, they offered the Corleone family one of their men as a hostage, with the understanding that if Michael didn't come back safe and sound, their man would die, and it would be big trouble for Sollozzo. But I cannot think of the name for the life of me. I don't know if Puzo would refer to them as the elusive "6th Family," but it is a possibility.