This is the 3rd in the Venice trilogy. Number two (Alchemist's Code) came out in mass market a week ago.
Here is a summary of The Alchemist's Pursuit:
Being even crabbier than usual, Maestro Nostradamus refuses a request from Violetta Vitale, the finest courtesan in Venice, to investigate the murder of her friend Lucia. But when another courtesan dies, and then another, the Maestro repents and sets Alfeo Zeno--his apprentice and Violetta's lover--to work gathering evidence. Their quest has hardly begun before the ruling Council of Ten intervenes, forbidding them to pursue the investigation.
Any normal detective would be balked, but the Maestro is the finest seer in Europe, and even the implacable Council of Ten cannot prevent him from seeking to foresee the next murder and sending Alfeo to catch the killer in the act. The hunt becomes ever more urgent and desperate as it seems that Violetta herself may be destined to be the next victim and Alfeo begins receiving mysterious occult help that he did not ask for and probably dare not accept.