Reference sources provide a broad or general introduction to a topic, and often provide links or "pointers" (e.g., bibliographies, references, authors) to other information sources. They are a great place to begin exploring your research topic, and may help you convert a very broad topic into a more focused, more specific research question. Specific reference sources include dictionaries and encyclopedias.
Formerly Grove Music Online, it offers full text of the 29-volume print edition with the added benefit of sophisticated search capabilities, one-click cross-referencing, and an ever increasing network of web-links to musical sites around the world.