You can read No More Good Girl by itself but Russian Mafia Tsar delves into who Kirill and Mia are and their love story, so No More Good Girl would probably make more sense if you read Russian Mafia Tsar first. However, I think, the story in NMGG doesn’t rely on information from Russian mafia tsar too much. For Mafia’s bloodline you definitely need to read no more good girl first, otherwise nothing will make sense.