Arms of Nemesis by Steven Saylor

Arms of Nemesis is a mystery set in ancient Rome at the time of Spartacus' slave rebellion. Detective Gordianus the Finder investigates the murder of a cousin of Marcus Licinius Crassus, the richest man in Rome, who wants to lead the army against Spartacus. Two runaway slaves are blamed for the murder, and Crassus wants to slaughter the whole household of slaves in revenge. Gordianus is sure they're innocent, but he has to prove it to Crassus' satisfaction in three days'...