Venus in Copper by Lindsey Davis
In the heyday of the Roman Empire, a small accounting error has left Marcus Didius Falco sharing a cell with a large rat. But, the Empire's most hard-done-by investigator is finally bailed out, and promptly accepts a commission to help a family of freed slaves fend off a professional bribe...This is a third instalment of the Marcus Didius Falco mysteries set in first century Rome, starring Anton Lesser.