Just Before Midnight (Meet Me at Midnight)
Suzanne Robinson

No one knows that Cheyne is working with Scotland Yard to catch a blackmailer plaguing London's wealthiest aristocrats. He'll need a daring young woman to help capture the culprit--but no gilded English lily will do. Mattie, with hair black as midnight and a fearless soul to match, is just perfect. But Cheyne doesn't know just how far Mattie will go to find the adventure she craves. And when he finally realizes how much she means to him, it may be too late to protect her....