See, for instance, Carsten Peter Thiede and Matthew d’Ancona, The Quest for the True Cross (New York: Palgrave, 2000). See Jan Willem Drijvers, Helena Augusta: The Mother of Constantine and the Legend of Her Finding the True Cross (Leiden: Brill, 1992) for a historically more reliable picture. See also http://www.roman-emperors.org/helena.htm.