Published primary sources for the history of art history and other disciplines in the eighteenth and nineteenth centuries are increasingly available on line. Not all digital versions are linked in WorldCat (still the first and quickest way to search). I find many obscure but essential sources by going directly to the online catalogs of the German libraries listed below. These libraries have their own digitization projects, providing excellent color scans and/or pdfs in viewers of varying convenience. Some also link to Google Books, allowing quick access to titles that are otherwise difficult to find. Munich and Göttingen have fast and relatively reasonable e-book on demand services. For sources in art and architectural history, Heidelberg has the best viewer, with excellent scans downloadable as a whole-text pdf or high-resolution jpg’s of individual pages. Berlin’s digitization project is not as far advanced as the others, but sources are becoming available. For secondary sources, the catalog of the Libraries of the Berlin Museums is invaluable.
Bayerische Staatsbibliothek, Munich: https://opacplus.bsb-muenchen.de/metaopac/start.do
Universitätsbibliothek, Heidelberg: http://www.ub.uni-heidelberg.de/helios/kataloge/heidi.html
Staats- und Universitätsbibliothek, Göttingen: http://www.sub.uni-goettingen.de/sub-aktuell/
Staatsbibliothek, Berllin: http://staatsbibliothek-berlin.de/
Bibliotheken der Staatlichen Museen zu Berlin: http://opac.smb.spk-berlin.de/