The brass queen

"By day, Constance Haltwhistle is a lady in possession of a fortune, very much in need of a noble husband. By night, she's the "Brass Queen," inventor of Victorian England's finest weapons. With the thoroughly unsuitable and utterly frustrating US spy J.F. Trusdale by her side, she sets out to find both a husband and a stolen invisibility serum to save her estate, and the world, in just three days"--