The Trouble with Moonlight by Donna MacMeans

Lusinda Havershaw has a secret - she has inherited the ability to become invisible in the moonlight. Unfortunately, this means that she cannot take part in the normal enjoyments of a young woman of the ton. She cannot attend the balls and house parties hosted by influential people for fear that her secret will be discovered.

James Locke has a secret, too - he's a British spy whose past is quickly catching up with him. He has been the British government's go-to guy when something needed retrieving from a locked safe. It is during one such mission, one to retrieve a list of spies with his name at the top, that he discovers something amazing - a necklace appearing to float out of the safe he has just cracked and take itself out of the house.

Following the path of that necklace brings James to Lusinda and he traps her into revealing her secret. He then determines to take advantage of her special abilities and train her in picking locks so she can aid him in retrieving the list. Neither of them imagined the intense attraction they would feel for each other or the danger they would be facing in their task. 

This is a fast paced book from the moment it starts with Lusinda's first 'retrieval' (as she and her aunt call it) and doesn't stop until the very end. Lusinda and James find themselves going from one dangerous situation to another, though the types of danger continually increase, from discovery to torture and death. Their feelings for each other seem to escalate along with the level of danger. It is only in their acceptance of each other, the gifts and the horrors of the past, that they can truly embrace a future together.