Read just a few books and you'll know a lot. WISEGUY by Nicholas Pileggi is one I would tell someone to read who wanted to learn about the mafia. It's what the movie Goodfellas was based on. It's got a lot of information about the day to day happenings of the mafia. UNDERBOSS by Peter Maas is also a good one. It's the biography of Sammy "The Bull" Gravano. It's good to learn about the higher ranks of the mafia, since he was underboss to John Gotti.

Those are just two books. There are many good ones about the mafia. Even the fiction books like the ones by Mario Puzo will teach you a lot.