The New International Biblical Commentary on John by J. Ramsey Michaels provides this information:
"This policy of amnesty is mentioned also in Mark 15:6 and Matt. 27:15 (by
the respective Gospel writers rather than by Pilate), but it is not mentioned
outside the New Testament. The reason for the silence of the Jewish sources
may be that the custom prevailed for a relatively short period of time--only
during Pilate's term of office, perhaps, or (at the most) over those decades
in which insurrections were more frequent and there were more political
prisoners than before. It was probably not a Jewish custom as such, but a
Roman concession to raise public morale" (page 323).
N.T. Wright, in Jesus and the Victory of God (page 546, note 30) writes:
"This account [in Matt 27:15, etc.] has been queried as part of the
evangelists' attempt to shift blame from Pilate to the Jews, but there is at
least the strong likelihood that it is historical: see, with full details
Brown 1994, 787-820."
"Brown" above is Raymond E. Brown. His work dated 1994 is The Death of the Messiah: From Gethsemane to the Grave. A Commentary on the Passion Narratives in the Four Gospels. New York: Doubleday.
One of the best study aids that I've personally found to illuminate John 18:39 is G.R-Beasley Murray's Word Commentary about John. His observations appear on page 334 of the commentary, and Murray directs readers toward the Talmud, Pesah.91a while he briefly discusses the significance of John 18:39.
Murray also mentions the JBL article by Charles B. Chavel, "The Releasing of a Prisoner on the Eve of Passover in Ancient Jerusalem." (JBL 60  273-78) as well as the discussions by J. Blinzler, C. K. Barrett, R. Schnackenburg, and F. F. Bruce. Check out Murray's commentary; I think you'll find it worthwhile.