This story is cute. It had a lot of potential. The main heroine and her family are very well developed as well as the hero. However, there were a few elements that I just couldn't let go.
First of all, she moves in with him without a female chaperone! This is Regency England. She is unmarried woman. This doesn't fly. First of all, she's so compromised and the servants think of her as a light skirt. Yea, yea. He gives his reasons why he wants her there but they don't work the cover story well enough and for a spy to be that sloppy, it ruins the illusion.
Other problems. Her sister's resentment and immaturity was so evident and so developed that when it came time for her epiphany, it didn't ring true and happened to suddenly. Marcus's development was also okay until the stupid twists at the very end. Really. And what was he going to do when the Russians realized the truth about the list.
Aside from that, it's a cute premise with a lot of potential. The main characters are developed, And the story is off the beaten path.