A Beautiful Disguise by Roseanna M. White

In Edwardian London, not all that glitters is gold as a lady and an intelligence officer’s secret mission take them from the city’s dazzling ballrooms to its covert intelligence offices.

Sir Merritt Livingstone has spent a decade serving the monarch in the field, but when pneumonia lands him behind a desk in the War Office Intelligence Division just as they’re creating a new secret intelligence branch, he’s intent on showing his worth. He suspects an aristocrat of leaking information to Germany as tensions mount between the two countries, but he needs someone to help him prove it, so he turns to The Imposters, Ltd. No one knows who they are, but their results are beyond compare.
Left with an estate on the brink of bankruptcy after their father’s death, Lady Marigold Fairfax and her brother open a private investigation firm for the elite to spy on the elite. Dubbed The Imposters, Ltd., their anonymous group soon becomes the go-to for the crème of society who want answers delivered surreptitiously. But the many secrets Marigold learns about her peers pale in comparison to her shock when she and her brother are hired to investigate her best friend’s father as a potential traitor.

Lady Marigold is determined to discover the truth for her friend’s sake, and she’s more determined still to keep her heart from getting involved with this enigmatic new client . . . who can’t possibly be as noble as he seems.