The New Grove Dictionary of Music and Musicians (20 Volume Set)

This is the big one -- 20 thick volumes packed full of information on musical history, composers, artists and more. It carries a big price tag, but it's an invaluable aid for the serious student or writer. Grove has several other dictionaries, opera and jazz among them, but if you can only handle one, this is the one to get. This text refers to the 20 Volume edition of this title.