Boehm, Barry W. Software Engineering Economics. Englewood Cliffs, NJ: Prentice-Hall, Inc., 1981. Section 22.2 of Boehm's book describes the original Delphi method and Boehm's creation of Wideband Delphi.
NASA, "ISD Wideband Delphi Estimation," Number 580-PROGRAMMER-016-01, September 1, 2004, http://software.gsfc.nasa.gov/AssetsApproved/PA1.2.1.2.pdf. This document describes a Wideband Delphi technique used by the NASA Goddard Space Flight Center.
Wiegers, Karl. "Stop Promising Miracles," Software Development, February 2000. Wiegers's paper describes a variation on the Wideband Delphi technique.