Platt, Elizabeth E.  “The Signet.”  In The Anchor Bible Dictionary, edited by David Noel Freedman, 3:829-30.  New York: Doubleday, 1992. [Israel/Kingship/Priesthood]

This is an informative article about the development of the signet ring.  It is not particularly well written as an article for a dictionary because she moves around and interjects seemingly extraneous information occasionally.  She explains that the signet ring probably came from Egypt as a scarab signet ring which gradually evolved over time.  The article also contains a list of the most prominent mentions of a signet ring in the Old Testament with brief but helpful comments about each one.  Interestingly, each one was in a position of considerable power.