Honestly, when I hear "mafia books exploring true crime," my mind goes straight to that grey area where fiction bleeds into research. There's a distinct difference between a novel using mafia tropes and a work trying to dissect actual criminal organizations.
You'll find a ton of 'romance' or family saga titles that use the aesthetics of organized crime, but if you want the real, gritty stuff, you have to leave the fiction section. I'd recommend starting with Selwyn Raab's 'Five Families.' It's a dense, journalistic history of the New York Mafia that reads like a sprawling, brutal epic. For something more narrative, 'Wiseguy' by Nicholas Pileggi is the foundation—it's what 'Goodfellas' was based on, and it lays out the day-to-day grind and moral decay in a way no novel ever quite captures.
Sometimes the most compelling true crime mafia stories aren't about the bosses, but about the systems. 'The Last Gangster' by George Anastasia about the Philly mob, or anything by John Dickie on the Sicilian Mafia. They ruin you for the glamorized fiction, which can be a good thing.