I like the basic concept of The Agency. A group of female spies in Victorian England! It sounds fabulous. A woman posing as a servant, a governess, for example, or a lady's maid or companion, could overhear and quietly collect a lot of things. So I came in ready to love this series.
Unfortunately, Mary Quinn goes directly from being informed that the Agency exists to arriving at her first job. She is apparently given "intensive" training in between, but we don't see even a moment of it. And I don't remember Mary thinking about her training much. It's a shame, because it's a huge missed opportunity. I would have very much liked to have seen that training. And since Mary does spend quite a bit of time screwing up in her first assignment, I'm left to believe that the Agency's training and selection procedures aren't quite as strict as they should be.
I don't doubt that Lee does know a fair bit about and has done some research into Victorian England. And times, it shows. But Mary herself reads more like a modern girl than a Victorian girl in a delicate situation. Her behavior (including arguing politics with a guest of her employer) just didn't ring true for the period. This won't bother some readers, but it did sometimes take me out of the story.
But I think those two things might not have bothered me as much if I liked Mary more. It's not that I disliked her. I was sympathetic towards her and her circumstances, but not attached to her. She just didn't stop to think quite as much as I like to see in my heroines. A shame, because I really did want to like this series.