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Kat Shehata has a trilogy. Russian Tattoo's.

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Bethany Kris has Guzzi duet but also writes various other Mafia books.

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Hayley Faiman has a Russian Bravata series.

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London Miller has a Russian Volkov series.

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Rachel van dyken Elite series has various Mafia factions, one of her books features Sergei but it's fits within the series so not really a standalone.

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Bethany Kris The Russian Guns series.

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Roxie Rivera Russian Protector series but the author hasn't finished the series due to serious health problems.
Each book is a separate couple but the last book drops hints about the next book which has not or may not ever be written.

You can try The Dominator series by D.D. Prince.
It is not about the Bratva though, more about italian mafia.