Boyd-Bowman, Peter, From Latin to Romance in Sound Charts, Georgetown University Press, Washington D.C. (1980).
Canfield, D. Lincoln, and Davis J. Cary, An Introduction to Romance Linguistics, Southern Illinois University Press: Carbondale and Edwardsville (1975).
Elcock, W.D., The Romance Languages, Faber and Faber: London (1960). First edition, a second edition exists but I have not seen it.
Hall, Robert A. Jr, External History of the Romance Languages, Elsevier: New York, Oxford, Amsterdam (1974).
Hall, Robert A. Jr, Proto-Romance Phonology, Elsevier: New York, Oxford, Amsterdam (1976).
Jackson, Kenneth, Language and History in Early Britain, University Press: Edinburgh (1953).
Posner, Rebecca, The Romance Languages, University Press: Cambridge (1996).
Russel, Paul, An Introduction to the Celtic Languages, Longman: London and New York (1995).
Wright, Roger, Latin and the Romance Languages in the Early Middle Ages, Routledge: London and New York (1991).