The tin princess - Philip Pullman

Jim Taylor, Sally Lockhart's oldest friend, is engaged as bodyguard
to a princess. But Crown Princess Adelaide of Razkavia is not
what you'd expect. She's the slum-girl Jim has been searching for
ever since she vanished ten years ago. Now her life is in mortal
danger, and together they must battle against unknown enemies in
a country where no one can be trusted. This dramatic story of love,
loyalty and adventure is the final novel in the quartet. Philip
Pullman's ever-popular, action-packed Victorian melodramas are
rejacketed for the bicentenary of Charles Dickens in 2012.