I. Unsigned Dictionary or Encyclopedia Article (Turabian 11.42):
 

Apel, Willi, ed. The Harvard Dictionary of Music.  2nd ed. Cambridge:  Belknap Press of
        Harvard University Press, 1969.  S.v. "Virtuoso."

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

Slonimsky, Nicholas, rev.  Baker's Biographical Dictionary of Musicians.  8th ed.  New York:
        Schirmer, 1992.  S.v. "Nordgren, Pehr Henrik."