Most of the stuff we know about "Barbarians" comes from either Roman or Greek historians anyway, so why not try "Comentarii de bello Gallico" by a guy called Caius Julius Caesar or "Germania" by another guy Publius Cornelius Tacitus?

www.forumromanum.org
http://www.forumromanum.org/literatu...ar/gallic.html
http://www.fordham.edu/halsall/basis...rmanygord.html