In Young Adult, but probably an older YA (for occasional sexual references), otherwise I'd recommend it for people running out of Nancy Springer's Enola Holmes mysteries. 
 Starts a little clumsily, but the story moves well after that. Mary Quinn is rescued from the gallows and offered a chance to be more. She joins the Agency and we follow her through her first case. Even though it's a very woman-powered story, Mary gets help from a young man, James. There's obviously a connection there which I assume will be followed up in another book.