The devil's looking-glass - Mark Chadbourn

1593: The dreaded alchemist, black magician and spy Dr John Dee is missing...
Fear sweeps through the court of Queen Elizabeth, for in Dee's possession is an obsidian mirror, an object of great power which legend says could set the world afire.
And so the call goes out to celebrated swordsman, adventurer and rake Will Swyfte: find Dee and his feared looking-glass and return them to London before disaster strikes. But when Will learns that the mirror may help him solve the mystery that has haunted him for years, the stakes become acutely personal.
With a frozen London under siege by supernatural powers, time is running out. Will is left with no alternative but to pursue the alchemist to the devil-haunted lands of the New World and the terrifying fortress home of mankind's ancient enemy, the Unseelie Court.
Facing an army of these unearthly fiends, with only his sword and a few brave friends at his back, the realm's greatest spy must be prepared to make the ultimate sacrifice - or see all he loves destroyed.