John Stambaugh, "The Ruling Class and its Clients" in The Ancient Roman City (Baltimore: Johns Hopkins University Press, 1988), pp.91-93 
Smith's Dictionary of Greek and Roman Antiquities (1875) s.v. "Nobiles"
Christopher Francese, "Nobilis" from Ancient Rome in So Many Words (New York: Hippocrene, 2007), pp. 58-60.