The Traitor's Smile by Patricia Elliott (Published by Hodder Children's Books, May 6, 2010)
Eugenie de Boncoeur has fled the violence of the French Revolution to find sanctuary in England at the home of her cousin, Hetta. At first, the two girls find themselves at loggerheads: Hetta can't understand Eugenie's preoccupation with clothes and appearance, and scorns her politics. Soon, however, they are drawn together by a shared sense of danger, for across the Channel waits the vengeful Pale Assassin, determined to claim Eugenie for himself. With her brother's life at stake, how can she refuse his dreadful bargain? But it will mean sacrificing her chance of love and returning to Paris in the grip of the Terror. Eugenie must now decide her destiny - with or without Hetta's help.
I had a bit of a hard time getting into the first book in this series, The Pale Assassin, because I found the main character to be really immature and helpless at first... however I did eventually get into the story and I want to find out what happens next after the cliffhanger ending! This is the UK release date, the first book did get a US release so hopefully this one will too, but in the meantime, if you can't wait, you can pre-order this book with free worldwide shipping from The Book Depository: http://www.bookdepository.com/book/9780340956779/The-Traitors-Smile