The Alchemist's Code — Dave Duncan

The legendary Maestro Nostradamus may be able to glimpse the ever-changing future, but even he cannot see the danger that is about to envelop him and his daring apprentice Alfeo when Nostradamus is hired to find a foreign spy by Venice's ruling Council of Ten. The only clues they can offer him are the spy's intercepted messages, encoded in a seemingly unbreakable cipher.

But Nostradamus soon detects evil influences working against him, and realizes the spy can only be caught by occult means. He turns to his able apprentice, the young swordsman Alfeo Zeno, whose unique talents may prove essential to unraveling the truth.