The plot is about the unexpected romance that blossoms between a yakuza member and someone unexpected. It's appealing because of the unique mix of action and romance.
I'm not aware of any extremely popular ones off the top of my head. However, in the broader genre of Japanese literature that touches on the yakuza and relationships, there might be some that fit this bill. Some authors might incorporate elements of yakuza life and love in their works, but it might not be a mainstream, widely - known sub - genre specifically titled 'yakuza in love novel'.