A Spy in the House by Y.S. Lee

Introducing an exciting new series! Steeped in Victorian atmosphere and intrigue, this diverting mystery trails a feisty heroine as she takes on a precarious secret assignment.

Rescued from the gallows in 1850s London, young orphan (and thief) Mary Quinn is surprised to be offered a singular education, instruction in fine manners — and an unusual vocation. Miss Scrimshaw’s Academy for Girls is a cover for an all-female investigative unit called The Agency, and at seventeen, Mary is about to put her training to the test. Assuming the guise of a lady’s companion, she must infiltrate a rich merchant’s home in hopes of tracing his missing cargo ships. But the household is full of dangerous deceptions, and there is no one to trust — or is there? Packed with action and suspense, banter and romance, and evoking the gritty backstreets of Victorian London, this breezy mystery debuts a daring young detective who lives by her wits while uncovering secrets — including those of her own past.
 
- Goodreads Summary