In the second of a series (begun with A Comedy of Murders, 1994), the setting is once again Renaissance Italy and the sleuths are Leonardo da Vinci and his more than worthy sidekick, the learned dwarf Niccolo. When a courier is savagely murdered and the (title) object in his keeping stolen, Leonardo, having fled French- occupied Milan, is summoned to Mantua to investigate whether those crimes were, as is suspected, the work of ``the driving force behind the courts of Italy . . . Women.'' He and the irrepressible Niccolo discover avarice and worse in the hearts of every Borgia and Borgia-like nobleman (and -woman)--as well as complicated intrigue lurking in every palazzo--during a journey that also leads to Venice during ``carnivale'' and to a climactic encounter in the Street of the Assassins. It takes a while for Herman to link his novel's opening episode with subsequent events and all characters, but once the plot gets rolling it gathers real momentum. His prose glitters with wit, featuring dozens of sophisticated conversational exchanges, and his knowledge of the period's art, architecture, geography, and social history seem impeccable. The thoughtful, cautious Leonardo is a completely believable creation--and Herman ingeniously employs his hero's knowledge of the anatomy to help solve the mystery. It's a virtual embarrassment of riches when we recognize graceful allusions to Guy de Maupassant and Sherlock Holmes. A delight in what we hope will be an ongoing series.