The Devil's Looking-Glass by Mark Chadbourn

The third swashbuckling, supernatural adventure featuring Elizabethan England's answer to James Bond, the dashing swordsman and spy, WIll Swyfte.
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1593: The dreaded alchemist, black magician and spy Dr John Dee is missing... Terror sweeps through the court of Queen Elizabeth, for in Dee's possession is an obsidian mirror, a mysterious 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 discovers the mirror may help him solve the mystery that has haunted him for years -- the fate of his lost love, Jenny -- the stakes become acutely personal. With a frozen London under siege by supernatural powers, the sands of time are running out. Will is left with no choice but to pursue the alchemist to the devil-haunted lands of the New World -- in the very shadow of the terrifying fortress home of England's hidden enemy, the Unseelie Court.
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Surrounded by 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.

This book is part of the "Swords of Albion" series.
Here are some other books from this series: