City of Magic by Avi (Ex-Library HC/DJ)
Midnight Magic Series #3
When King Claudio sends Mangus the Magician and his faithful servant, Fabrizio, to Venice to steal a manuscript that explains a magical method of making money, they must succeed in their mission or risk death. The manuscript is key to obtaining great profits for the king―and a secure future for Mangus and Fabrizio.
But Venice in 1492 is a dangerous place, full of foggy canals, cunning informers, and harsh punishments for those who steal its secrets. Before long, Mangus is snatched away into prison, and it’s up to Fabrizio and his secretive new friend, Bianca, to navigate the city, find the manuscript before their enemies do, and keep Mangus alive!