Mina Wentworth and the Invisible City — Meljean Brook

After Mina is called in to investigate the murder of an aristocratic bounder, her husband Rhys--known to the world as the Iron Duke--must overcome his fear for his new bride before his need to protect her tears them apart. But when she invites him to join her at the murder scene, a search for a mysterious killing machine puts her in more danger than ever before...a danger that leads straight back to Rhys.

Previously published in "The Iron Duke."