You could start off with "Complete Idiot's Guide to the Mafia" (note: the "complete idiot" thing in the title is an ironic joke in case you aren't aware). It has a couple of mistakes in it, and the "current" information is a bit dated, but it is a good primer on everything.

After that, check out "Blood and Honor" and "Wiseguy" to get a good representative view from inside a family from the soldier and associate perspectives, respectively. You'll get a thorough sampling of the rackets, scams, violence, betrayal, politics, and the like from those two books.

These three books will give you a good foundation of knowledge on which to build.