Sometimes a Rogue by Mary Jo Putney

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Oh. I liked this book. I've been waiting to read Sarah's story since her twin got married in Loving a Lost Lord. And Sarah starts out the book as a heroine.
She's walking with her sister, Mariah, a duchess about to give birth. They hear some men planning to kidnap Mariah and Sarah bravely takes her place. She's a spinster with nothing to lose while Mariah is living a very happy life.
It's a good thing that Rob Carmichael is just arriving on the estate. He's a Bow Street Runner (though noble born) and is still recovering from his some-time bedmate falling in love with, and marrying, a man that Rob doesn't particularly like (another good book, No Longer a Gentleman.) He thought they were in a committed relationship but the same wasn't true for the woman with whom he was involved.
But now he gets his HEA. If only they can escape the men who have kidnapped Sarah.

I loved that Sarah could ride horses so well but really disliked that Putney kept calling her a "tomboy who ran wild." It's great for the first description of Sarah as a girl, but after about the third time... eh. It was also interesting to me that Adam's aunt came back into the picture, though it was plain she was the villain but I am very happy that they killed her off.
I also enjoyed Rob's grandmother. She is cruel to him at first but then realizes that she is backing the wrong team by continuing to believe the lies of his dead half-brother and quickly starts to help Rob start cleaning up the mess left by the last two earls.
A major relief was that Rob's first love didn't turn out to be alive. The bastard daughter was an interesting spoke but I was so afraid that her mother was going come back and I just didn't see how Sarah could compete or how Rob could turn her away without being a complete jerk, even if she had "changed."

I loved the character development of the major characters and the minor characters were also fun. The villain was... maybe an unnecessary wrinkle but it was minor in an otherwise highly enjoyable book.