Her Clockwork Heart

Nathaniel Dennison is an investigator with the Royal Intelligence Office. Because of his knack for solving puzzles, he's assigned a case involving a string of missing scientists. During his investigation he crosses paths with Trixie, the only woman to ever tempt him into considering marriage. Despite the bruising his heart took when she left, he can't leave her to search for her missing brother on her own.
Beatrix "Trixie" Wadeworth is desperate to find her brother. So desperate that she breaks into the Royal Intelligence Office in London. Getting caught is inconvenient. But getting caught by the one man she never stopped loving may actually be fortuitous. It feels like a lifetime since she last saw Nathaniel but it only takes seconds for those feelings to come rushing back. Danger follows Nathaniel and Trixie from London to Edinburgh and back as they search for clues.
In the end they find there is more than one mystery to solve. How they ever lived without each other may be the greatest of them all.