A single coin that is related stylistically to those of the sole reign of Egica indicates that a king of the name of SVNIEFREDVS seized power in Toledo at some point in these years, and for long enough to have the mint start issuing in his name.
- Thompson, E. A. The Goths in Spain. Clarendon Press, 1969.
- Collins, Roger. Visigothic Spain, 409–711. Blackwell Publishing, 2004.
- Livermore, Harold. Twilight of the Goths. Intellect Books, 2006.
- Montenegro, Julia; Castillo, Arcadio del. "Theodemir's Victory over the Byzantines in the Joint Reign of Egica and Witiza: A reference by the Chronicle of 754". Byzantion, 74, 2 (2004), pp. 403–415.
- Collins, Visigothic Spain, 107.
- Thompson, The Goths in Spain, 244: "Sunifredus, comes scanciarum et dux".
- (in Spanish) J. A. Castillo Lozano, "La enigmática figura de Suniefredo a la luz de sus emisiones monetales", Revista Numismática Hécate Nº 2, 2015.
- (in Spanish) Pío Beltrán, "Iudila y Suniefredo, reyes visigodos", Ampurias, 3, 1941.