The Brass Queen by Elizabeth Chatsworth

She knows a liar when she sees one.
He knows a fraud when he meets one.

In a steam-powered world, Miss Constance Haltwhistle is the last in a line of blue-blooded rogues. Selling firearms under her alias, the “Brass Queen,” she has kept her baronial estate’s coffers full. But when US spy J. F. Trusdale saves her from assassins, she’s pulled into a search for a scientist with an invisibility serum. As royal foes create an invisible army to start a global war, Constance and Trusdale must learn to trust each other. If they don’t, the world as they know it will disappear before their eyes.

If you like the Parasol Protectorate or the Invisible Library series, you’ll love this gaslamp fantasy—a rambunctious romantic romp that will have you both laughing out loud and wishing you owned all of Miss Haltwhistle's armaments.