The Queen at War by K.A.S. Quinn
In twenty-first century New York, Katie Berger-Jones-Berg is a little concerned. She's seeing people in old-fashioned clothes who have a worrying habit of disappearing. Then she receives a mysterious note...and is sent hurtling back through time!
In ninteenth-century London, Queen Victoria is on the throne and England is on the brink of war with Russia. For some reason, everyone's looking to Katie to save the day.
But for a traveller in time, Katie's is fast running out...