"A Spy in the House" is the first in a series of four YA novels set in London in the 1850's. Once you accept the notion that an agency of educated women undertake clandestine investigations, you can enjoy a fast-paced little mystery thriller. The descriptions of London at the time of the Great Stink are vivid.
The lead character is Mary Quinn, an orphan who was saved from the hangman, educated to be a teacher, and then initiated into the secrets of the agency. In her first job, she takes the post of lady's companion in the house of a wealthy merchant. Angelica, the daughter of the house, is not pleased to be saddled with a prim guardian. But Mary's real job is to ferret out information about Mr. Thorold's mysterious and probably illegal business dealings. Along the way, she encounters James Easton, an up-and-coming engineer, who is trying to sniff out any scandals about the Thorold family before his brother proposes marriage to Angelica. James and Mary are quite literally thrown together. They soon find that it is to both their advantage to cooperate. The plot zips along with some interesting twists.
This book reminded me of the Sally Lockhart series from Philip Pullman's early years, particularly "The Ruby in the Smoke". If you want a fast-paced historical mystery with a light touch, I recommend this.