Fred L. Horton, Jr., in a monograph, discusses traditions concerning Melchizedek in the Bible and extra-biblical literature such as the Dead Sea Scrolls and Josephus.
Fred L. Horton, Jr., The Melchizedek Tradition: A Critical Examination of the Sources to the Fifth Century A.D. and in the Epistle to the Hebrews (Society for New Testament Studies Monograph Series 30; Cambridge: Cambridge University Press, 1976)
[From back cover]
This monograph seeks to answer the question why Melchizedek, who is a minor figure in the Old Testament, is selected by the author of the Epistle to the Hebrews to represent the priesthood of Christ. In the course of his study, Professor Horton surveys the traditions about Melchizedek from the Old Testament period, in Philo, Josephus and Qumran, to the later sources in Rabbinical and patristic writings and the Gnosticism. The book concludes with a critical examination of the claims made for the dependence of Hebrews on the Qumran figure and tradition. The author finally rejects this interpretation in favour of a source and background for Hebrews in an independent and largely original interpretation of the Old Testament passages.