This book is mistitled. He wasn't a Barbarian and she didn't want to be his mistress.
she wanted to be his wife, having loved him since she was a child.
I don't usually read or like books that go into graphic detail about sex; it's gratuitous and unnecessary. Not in this book though, it's an integral part of the story.
The four main characters are all emotionally scarred by sex in their past and this is a story of discovery, awakening, redemption and love.
Oh, and revenge, which by the time it comes has you wanting to spit in his eye as he dies.