Midnight in Sicily by Peter Robb is a fascinating account of Sicily in general and Palermo in particular. Robb lived in both Naples and Palermo, and he writes in detail about the food, culture, and criminal underworld of each. He presents extensively researched material on La Cosa Nostra, the politics of Italy and Sicily, the history of pasta, Lampedusa's The Leopard, and the rituals of the tuna catch in western Sicily.      

The Leopard, by Tomasi di Lampedusa, is itself worth reading before – or even during – a visit to Palermo: a wonderful novel filled with an atmosphere of decaying grandeur that still pervades much of Palermo even today.

And, on a lighter note, Sweet Honey and Bitter Lemons by British food writer Matthew Fort presents an entertaining but insightful tour of Sicily and its food, including a thoughtful chapter dedicated to Palermo. 

Palermo City Guide by Adriana Chirco is available in English and is a good guide to the city and helpful in planning your trip. Also illustrated with photos.