I've been thinking about running a campaign where the party works for the Xanathar organization or some other organized crime group in the forgotten realms. Are there any books that could give me some inspiration on how to run a game like this? Does anyone have any thoughts on a mafia D&D game? 

Certainly!

My table actually had a few sessions where we were smugglers hired by the Myriad criminal organization…a “Critical Role” crime syndicate… to smuggle some valuable objects into the city…while only some of us were affiliated on a personal level; it was a good example of being in a “mafia” campaign.

Some things to consider:

The players should answer to a “boss”…that is, if they start out as low-level gangsters, they need to answer to someone within the mafia itself.

This could be a low-level enforcer who is barely ranked higher than the players themselves…or it could be a real nasty boss, whose made a big name for themselves in the criminal underworld….perhaps they’ve been entrusted with nearly all criminal activity in a given city. They should be smart, skilled, and charismatic.

It’s usually a good idea to give a “boss” something to distinguish them…something that freaks out the party, or a nickname that highlights why they’re dangerous.

Example: my crime boss is Korvic the Throatcutter, a scoundrel who deals with traitors & double-dealers in his syndicate pretty much the same way…he cuts their throats, and sticks their tongues in jars (which he displays in jars around his office as a warning).

Sometimes he even eats tongues for meals while holding meetings (these are actually pig tongues he purchases from a local butcher; but he doesn’t tell anybody that).

Next, you want to have an explanation why the players are working for the mafia…do they owe them a debt? Have they fallen on hard times? We’re they raised by the syndicate and consider them like family?

Then, you want to give the players jobs…if you’ve seen thieves guilds in fantasy games; there are types of jobs for every type of players: intimidate a local shopkeeper into paying the mafia the money they owe them….smuggle some contraband into the city…or smuggle some people “out” of the city…intimidate the judge into declaring a criminal to be innocent…threaten witnesses….deal with local ruffians to get the people on your side…figure out if a rival gang is killing your members, or if something worse is going on…run from the city guards, or try to get this one particular detective from ruining your operation.

Which brings up a good point: who is the bad guy in a mafia campaign?

Is it the nobility, who want to root out all the criminals in their city? Is it a rival gang?

Maybe the king or emperor has hired their network of spies & assassins to wage a secret war with your organization…

…or is the threat magical? A group of cultists or wizards, who can’t be bribed or persuaded to leave you alone? (check out “The Lies of Locke Lamora” for a good example of a secret thieves guild with this plot line)

Finally…figure out your hideout: where is your base of operations?

Is it a tavern? An underground sanctuary? Is it on a boat? A corrupt noble’s private estate?

Finally…make memorable NPC’’: I mentioned the “boss”, but every good crime boss has a loyal consigliere to watch their back, and occasionally do their dirty work.

Al Swearengin from “Deadwood” had Dan Doherty, after all, whenever he needed to have someone killed…then he had someone to mop up the bloodstains, a pig farmer to feed the corpse to, and a doctor to stitch up his people.

Think if this mafia of yours is like a family, watching each other’s backs…or if everyone is a backstabber, only in it for the money.

 Another question is what is the organization of this mafia like?  Is it a meritocracy where organization is based on skill?  Is it a race-based organization made up of all elves, or dwarves, or some other race?  I tell you, nobody expects the gnome mob.

Pick up Dragon Heist. It has a lot of gritty flavor as well as specific info on the Xanathar Guild and the Zhentarim.