A Spy in the House by Y. S. Lee

Mary Quinn is only twelve when she’s sentenced to hang for theft in Victorian London. Rescued at the last moment, she’s offered a surprising new life—as part of an all-female detective agency posing as maids, governesses, and companions to uncover society’s deepest secrets. When Mary’s first assignment leads her into the home of a wealthy merchant suspected of smuggling, she must balance deception, danger, and her own mysterious past. Smart, suspenseful, and richly atmospheric, this is the first novel in The Agency series.