Stealing the Heiress by Samantha Holt

She was looking for adventure…
Lady Rosamunde Stanley just never expected in the form of a kidnapping.
Well, an almost-kidnapping. The dastardly man only went and took the wrong woman!
But she soon discovers the dark, handsome man is not quite the terrible rogue she thinks him to be and she decides to put his experience to use. Her uncle has gone missing and no one believes her when she says he’s in trouble. Can she persuade Russell to help?
Russell might have kidnapped her, but he has no intention of keeping her. Even if the heat between them is driving him out of his wits and her crazy family makes him long for things a battle-hardened orphan like him should never want.
Their societal divide isn’t the only reason to keep his distance. Rosamunde is a bloody knife-carrying, picklock-wielding madwoman.

A far too appealing madwoman…
He’s not the only one struggling. Rosamunde is finding it just as difficult to fight the attraction, despite knowing it is most likely her over-active imagination leading her down this path. Somehow, she must keep her head around the most exciting man she’s ever met.