Bibliography and further reading about early Germany:
Europe, by Jo Ellen Moore (1999). For kids, a basic geography of modern Europe, including information about animals.
Germany in Pictures, by Jeffrey Zuehlke (2003). A basic geography of Germany for kids, with pictures. It's about modern times, not history, but the geography's more or less the same.
The Ancient Celts, by Patricia Calvert (2005). Covers all aspects of ancient Celtic society (in Germany and elsewhere), using archaeological evidence to show the lives of farmers, soldiers, and craftspeople.