After you get a good overview of the Mob in general,you could also pick up some specialized books for a more finely tuned perspective. Among many others,Donnie Brasco by Joe Pistone is a great look into the day to day life in the Bonanno's and is far better than the film. I would also check out The Last Mafioso by Ovid Demaris. It's the story of Jimmy(TheWeasel)Frattiano,and covers a lot of ground.I like to haunt Flea Markets,Half Price books,Thrift Stores,etc. for stuff that the Library doesn't have. Also if you go into Google Books,you can enter search terms and specify books with previews available, There are also numerous online archives that will keep you busy for a long time.
Once you start,it's just like potato chips.