I think I would give this book more than 5 stars if I could. While I've enjoyed all the books in the Maiden Lane series, Duke of Sin is just excellent. 
 Valentine Napier, Duke of Montgomery, has appeared in other of the books. He is a rogue of the worst sort, formed by childhood experiences. He is careless, witty, charming, and nude a lot. And fascinated by his new housekeeper, Bridget Crumb. 
 Bridget is the illegitimate daughter of a member of the aristocracy who has risen in the service ranks by her merit and industry. She understands Val in a way that others don't. She doesn't cut Val any slack, which he actually likes, and is a perfect foil for him. 
 Val is outrageous and pretty much says and does what he wants which is part of his charm. This is Elizabeth Hoyt at her finest. The writing is just terrific. I did find the ending predictable, but the story is so good that it didn't matter. I can't wait for the next book in the series.