Apel, Willi, ed. The Harvard Dictionary of Music.  Cambridge:  Belknap Press of Harvard
        University Press, 1969.  S.v. "Opera" by Donald J. Grout.

Randel, Don Michael, ed.  The New Harvard Dictionary of Music. Cambridge: Belknap Press of
        Harvard University Press, 1986.  S.v. "Opera" by Charlotte Greenspan.

Sadie, Stanley, ed. The New Grove Dictionary of Music and Musicians.  Washington, D.C.:
        Grove's Dictionaries of Music, 1980.  S.v. "Virtuoso" by Owen Jander.