Books in the Electra McDonnell series

"Safecracker Ellie McDonnell hasn't seen Major Ramsey--her handsome but aloof handler in the British government--since their tumultuous mission together three months before, but when she hears about a suspicious robbery in London she feels compelled to contact him. Together they discover that a rash of burglaries in the city all lead back to a hotbed of spies in the neutral city Lisbon, Portugal, and an unknown object brought to London by a mysterious courier. As the thieves become more desperate and their crimes escalate, it becomes imperative that Ellie and Ramsey must beat them at their own game. Fighting shadowy assailants, enemy agents, and the mutual attraction they've agreed not to acknowledge, Ellie and Ramsey work together to learn if it truly takes a thief to catch a thief"--