"Palermo : guide della citta e dei dintorni" is an excellent profile of Palermo. This is one of the most exhaustive and approachable resources around. An English translation exists (according to OCLC), but I have never seen it. The book profiles the various quarters of Palermo and has short and descriptive details about many buildings, churches and sites in the city. It is also filled with maps of the city and drawings of the buildings. There are also sections on a few of the outlying areas of Palermo (Monreale, in particular). This book is nearly impossible to find the in the United States, but, if you head to Palermo, you will find it in almost every bookstore in the city.