The Iron Hand of Mars — Lindsey Davis

When wild Germanic troops in the service of the Empire begin to rebel, and a Roman general disappears, the worried Emperor Vespasian turns to the one man he can trust: Marcus Didius Falco, Imperial Rome's answer to Columbo.

To Falco, an undercover tour of Germania is an assignment from Hades. On a journey that only a stoic could survive, Falco meets with disarray, torture, and murder. His one hope: in the northern forest lives a powerful Druid priestess who perhaps can be persuaded to cease her anti-Rome activities and work for peace. On the other hand, Falco may just be more grist for the mill....