First, I recommend JabRef, an open source bibliography reference manager using BibTeX as file format. Second, there is an easy to use JabRef plugin for the Union Catalogue (GVK) of several German federal states participating in the so-called Common Library Network (GBV).
For JabRef see http://jabref.sourceforge.net/.
For downloading the JabRef-GBV-plugin see http://jabref.tempelb.de/.