Boy there aren't a lot out there specifically on Perugia. I found one guidebook reviewed on the Web but it is older (1993) and in Italian. http://www.ku.edu/history/index/europe/ancient_rome/E/Library/Reviews/

GUIPER.html

It's by Massimo Montella, editor, principal writers: Giovanna Chiuini (architecture and the city), Francesco Federico Mancini (art history), Simonetta Stopponi (Etruscan & Roman archaeology). Perugia. Elemond Editori Associati, 1993.

Amazon lists this one by Mancini in English as used only (out of print): Mancini, Francesco Federico, Perugia : historical and artistic guide, Italcards ; W. Pellegrini (distributer) Abebooks.com http://www.abebooks.com/ which lists books from many dealers and small shops lists a few older books including these:

Santini, Loretta, Perugia: new colour guide with city map Plurigraf, 1990.

Rowdon, Maurice, The Companion Guide to Umbria; Assisi , Perugia, Spoleto, London: Collins, 1969.

You might check your public library for this last one. Mine has it.

Frances Toronto, Canada