City of magic

Mangus the Magician and his thirteen-year-old servant, Fabrizio, are commanded to go to Venice, on pain of death, to find a book by Friar Luca Pacioli that reportedly contains a magical means of making money; but Venice is a strange and dangerous city, and someone else is also after the book--then Mangus is arrested, and taken to the prison, and Fabrizio and Bianca, an orphan he has met, must find a way to secure the book, save Magnus, and themselves.