IMSLP is a community-built digital library, consisting primarily of scores that are in the public domain.
Sibley Music Library at Eastman also has a digital library of scanned scores that are in the public domain.
Music & Performing Arts (Alexander Street) is a collection of cross-searchable databases that include text, streaming audio and video, and digitized scores. Search the databases together as well as individually for the most reliable results.
medici.tv is a video-on-demand database that contains performances from classical music festivals all over the world, as well as documentaries, master classes, etc.
DRAM is a streaming audio database of music from New World Records and other labels.
Met Opera on Demand delivers instant access to past performances of the Metropolitan Opera's Live in HD series, as well as classic video and audio performances, including radio broadcasts dating back to 1936.
Start with Oxford Music Online, which contains several music reference sources, to get an overview of a topic. Consult relevant articles and their bibliographies. For topics in world music, Garland Encyclopedia of World Music Online is also a good place to start.
Keep in mind, though, that reference sources like this are not continuously updated. Search literature indexes like RILM and Music Index Online (separately or simultaneously) for more recent books, book chapters, articles, and dissertations.
Search the Library Catalog (for books) and Articles & Full Text (for articles) to locate specific items cited in bibliographies, and to see what else may be available.
*** (If you are looking for a chapter from a book, search the title of the book rather than the title of the chapter. If you are looking for a journal article, search the title of the article, rather than the title of the journal.