I agree about the research and all the rest of it, but why write about something you have no personal knowledge of? Why not write what you know?
Mafia stories have been truly flogged to death, I doubt if there is still a market for them. We’ve moved on, the current trends are not about some daft crime organisation that hardly exists nowadays. Law enforcement has come a long way since police commanders could be bribed by some greasy Italian gangster; telephone intercepts and satellite observation have robbed those ancients of any credibility.
It was a romantic dream of years ago, Palermo and Omerta, nowadays they start singing as soon a some rookie cop slaps a hand on their shoulder. The informants don’t need protection any more, it’s the other way round, the few remaining gangsters need protection from the army of informants.
If you want to write about modern criminals, the libraries are just starting to fill up with Wall Street and oil wars. And terrorists, of course.
Sorry, I’ve just realised, if you’re writing historical fiction from hundreds of years ago, then you should research the Mafia.