Grove's Dictionaries of Music, 1984. Contains excellent bibliographies and illustrations. A good source of historical information about instruments appearing in works of art. The New Oxford Companion to Music . Oxford University Press, 1983.
Articles vary in length and no bibliographies are provided.
Randel, Don Michael. The New Harvard Dictionary of Music . The basic dictionary of terms in music. Be sure to note the bibliographies at the end of articles. Facts on File, 1987.