Cocktails & Chloroform by Kelley Armstrong

For modern-day detective Mallory Atkinson, being trapped in the body of a Victorian housemaid means overcoming endless obstacles. Her current challenge is winning over the suspicious young parlormaid, Alice. Mallory’s plan starts with teaching the girl to make Molotov cocktails, which is a perfectly valid science experiment and not at all a desperate ploy to gain Alice’s attention. Before the lesson can begin, though, Alice receives a letter that has her slipping off in the night. Concerned for her safety—and naturally curious—Mallory follows.

Mallory finds Alice at what seems like a simple dance hall, watching young men and women flirting and whirling in pretty dresses and dapper suit-coats. But nothing here is what is seems, and what starts as a simple surveillance exercise turns into a full-scale spy mission with Mallory’s boss, Dr. Duncan Gray, at her side. Before the evening is done, those Molotov cocktails are probably going to come in handy.

Note this is not a full-length novel. It’s a novella set after The Poisoner’s Ring.