The Jupiter Myth (A Marcus Didius Falco Novel), Lindsey Davis

For Falco, a relaxed visit to Helena's relatives in Britain turns serious at the scene of a downtown murder. The renegade henchman of Rome's vital ally King Togidubnus has been stuffed head-first down a barroom well - leading to a tricky diplomatic situation which Falco must defuse. One murder leads to others. Londinium now has a forum and an amphitheatre; the town is a magnet for legitimate traders - and for criminals from Rome.