Death Below Stairs by Jennifer Ashley

Victorian class lines are crossed when cook Kat Holloway is drawn into a murder that reaches all the way to the throne.
Highly sought-after young cook Kat Holloway takes a position in a Mayfair mansion and soon finds herself immersed in the odd household of Lord Rankin. Kat is unbothered by the family’s eccentricities as long as they stay away from her kitchen, but trouble finds its way below stairs when her young Irish assistant is murdered.

Intent on discovering who killed the helpless kitchen maid, Kat turns to the ever-capable Daniel McAdam, who is certainly much more than the charming delivery man he pretends to be. Along with the assistance of Lord Rankin’s unconventional sister-in-law and a mathematical genius, Kat and Daniel discover that the household murder was the barest tip of a plot rife with danger and treason—one that’s a threat to Queen Victoria herself.