The Agency series

A Spy in the House (first in the Agency series) by Y.S. Lee

Rescued from the gallows in 1850s London, young orphan and thief Mary Quinn is offered a place at Miss Scrimshaw's Academy for Girls where she is trained to be part of an all-female investigative unit called The Agency and, at age seventeen, she infiltrates a rich merchant's home in hopes of tracing his missing cargo ships.

Catch this series from the beginning:

1. A Spy in the House
2. The Body at the Tower
3. The Traitor and the Tunnel