So, I started watching The Sopranos recently (see review below), and having enjoyed it, I decided to go to the source, the well-spring of all mob stories, The Godfather , by Mario Puzo.
Wow. Let me say that again: Wow!
Written in 1969, this book still stands up today - it's fast, exciting, packed with interesting and detailed characters, and hell, just tells a rollicking good story.
As a novelist, what I liked most about the book was Puzo's wonderful digressions into little mini-stories about minor characters. For instance, the substory of the non-Mafia Italian who murders some businessmen who set him up for a corporate crime (he is sentenced to the electric chair; so the wily Godfather asks him to confess to a separate murder committed by his own son, in exchange for looking after the man's family after his execution).
If you haven't read it, go and out and read The Godfather today. You won't be disappointed (more than than: don't watch the movie until you've read the book; some parts of the-excellent-film don't really make sense unless you've read the novel).