KVK (Karlsruher Virtueller Katalog)
It is not a national library catalogue. However, you can search in many german, primarely university libraries, library catalogues at once. You can also search in several foreign national library catalogues. A total of 75.000.000 titles are searchable.
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This database is free to use even for none HH-affiliated staff and students. However, there may be restrictions in terms of full-text content.