I read this as part of a book club, and overall I enjoyed it. I believe it is listed as a YA book, but I think it could be appreciated by a wider audience as well.

Set in 1850s London, Mary Quinn, a poor 12 year old orphan, is sentenced to hang for stealing when she is rescued by the headmistress of a very selective academy for girls. She is educated, both intellectually and socially, and at 17 is asked to be a member of The Agency, a secret spy group made up of young women from the academy who assist Scotland Yard gathering evidence for arrests.

She insinuates herself as a lady’s companion into the home of a wealthy shipping merchant who is suspected of insurance fraud among other things. She has various adventures and is aided by new friends in her mission. There are some twists and turns, and the villain(s) become fairly obvious, but the unique idea behind the academy and Mary herself kept me interested.