One of the most interesting and useful tools for research is the metasearch engine Karlsruher Virtueller Katalog (KVK) of the Karlsruher Institut für Technologie. This was developed from the Institue library in 1996 and continues to expand and incorporate updates continuously.
The metasearcher allows the search of references in a huge group of libraries in Germany, Austria and Switzerland as well as most national libraries and other consortia in several countries. Especially useful is the section on digital libraries such as Europeana.
An important feature is the possibility of searching for references in second-hand bookstores on the Internet such as Abebooks, Amazon, …
This was one of the first tools that I started using for my research in the master’s degree and since then it is essential for any musicological theme. A must in the favorites.