The iron hand of Mars - Lindsey Davis

AD 71. Germania Libera: dark dripping forests, inhabited by bloodthirsty barbarians and legendary wild beasts, a furious prophetess who terrorises Rome, and the ghostly spirits of slaughtered Roman legionaries.

Enter Marcus Didius Falco, an Imperial agent on a special mission: to find the absconding commander of a legion whose loyalty is suspect. Easier said than done, thinks Marcus, as he makes his uneasy way down the Rhenus, trying to forget that back in sunny Rome his girlfriend Helena Justina is being hotly pursued by Titus Caesar. His mood is not improved when he discovers his only allies are a woefully inadequate bunch of recruits, their embittered centurion, a rogue dog, and its innocent young master; just the right kind of support for an agent unwillingly trying to tame the Celtic hordes.