Perhaps a bibliographic avenue of research on the paschal candle would be looking for references to the
Carmen paschale aka
Exultet. I know that there had been lots of work done on Beneventan liturgy's development of the hymn (with mansucript illustrations) but (sorry) I don't have it at the ready. There are some references in A. G. Martimort,
The Church at Prayer and J. A. Jungmann,
Pastoral Liturgy.
Maureen A. Tilley
Visiting Professor of Theology
Fordham University
113 W. 60th Street
New York, NY 10023
212-636-6369