Oxford Music Online is an important reference source; it includes Grove Music Online, a prominent music encyclopedia that makes a great first stop for any music-related research. Unlike many encyclopedias, Grove has entries written by experts in each topic. Some of the best entries are the composer articles, which include biographical and stylistic information, specific musical details and observations, and lists of the composer's works and writings. In addition to basic search functions, Grove allows you to browse the composer biographies by criteria like geography and time period, so it's useful for learning about new composers as well as searching for information about composers or topics you already have in mind.
