The Iron Hand of Mars

When Germanic troops in the service of the Empire begin to rebel, and a Roman general disappears, Emperor Vespasian turns to the one man he can trust: Marcus Didius Falco, a private informer whose rates are low enough that even the stingy Vespasian is willing to pay them.
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. Which Falco is eagerly hoping for as, back in Rome, the Titus Caesar is busy trying to make time with Helena Justina, a senator's daughter and Falco's girlfriend.
Lindsey Davis' historical mystery Iron Hand of Mars is a "Seamless blending of humor, history and adventure" (Publishers Weekly).