I wish there was a more straightforward answer to this! The title 'Mafia's Princess' pops up a lot, but it's actually used for a bunch of different books by different authors, so the plot changes depending on which one you've stumbled across.
The most common version I've seen is an arranged marriage / mafia romance. Usually, it's about the daughter of a mafia boss (the 'princess') who's forced into a marriage with a rival family's son or a powerful, dangerous man to settle a feud or forge an alliance. The core tension is her navigating this hostile, gilded cage—dealing with a cold, possessive husband who might secretly protect her, threats from outside enemies, and her own struggle between loyalty to her birth family and her new, unwanted situation. Think lots of forced proximity, enemies-to-lovers vibes, and dark glamour.
If it's not that one, it could be a story where she's hidden from the mafia world entirely and suddenly discovered, or a tale about her fighting to take over the family business herself against all odds. You really gotta check the author and blurb to nail it down.