Of Love and Evil by Anne Rice

Anne Rice's magnificent Songs of the Seraphim series continues with another metaphysical adventure full of angels and assassins.
Barely recovered from his previous divine mission, former contract killer Toby O'Dare is once again summoned by the angel Malchiah to investigate the poisoning of a prominent nobleman and stop the haunting of a diabolical dybbuk. Together, they travel back to fifteenth-century Italy--the age of Michelangelo, the Holy Inquisition, and Pope Leo X--and this time Malchiah has Toby pose as a lute player sent to charm and calm this troublesome spirit. But Toby soon discovers that he is in the midst of plots and counterplots, surrounded on all sides by increasingly dangerous threats as the veil of ecclesiastical terror closes in around him.