Irene has already drawn our attention to Caesar's description of the Germans (scroll down to paragraph 6.21 etc.)
Strabo has a geographical description of Germany followed by a second part, mainly about the Cimbri. Pliny's Natural History has a geographical section on Germany.
LacusCurtius has the section on Germania Magna from Ptolemy's Geography in Greek, Latin, and English with a map. The next sections on Raetia, Noricum and Panonia Superior are not up yet, but Pannonia Inferior (English text only) is.
