I learned a fair amount about the less well-known areas before I ever left home. I took a lot of books out of the library, but these are the ones I kept by my side and treasure.
If you are going to Italy, and you care about food, you must get Osterie & Locande D'Italia A guide to traditional place to eat and stay in Italy. I can't speak for the places to stay, but some of the best meals I had were places described in amazing detail in this Slow Food book. I yanked out the section on Campania and took it with me. It is by no means comprehensive, but the places it designates are winners and the detail on what to order the stories about the food and places and the people are fantastic.
The next book was my bible, The Food and Wine Guide to Naples and Campania. Carla Capalba is a thorough researcher and lovely writer. She spent years getting to know the people, places and stories behind the food. Her passion for the region comes through and I learned so much from her book I feel like sending her a personal thank you.
Faith Willinger's most recent book, Adventures of an Italian Food Lover with recipes from 254 of my very best friends is a charming book. It's filled with sweet watercolor portraits and of course, Italians and recipes. The section on Southern Italy and the Islands gave me not only names and addresses, but also a great feel for the people and the land. It's a storytelling book with recipes.
Upon my return, I really enjoyed perusing My Love for Naples The Food, The History, The Life is a really dear cookbook told by Anna Teresa Callen who was born and raised in Naples. The title says it all, really. Her recipes are very personal and so are the stories behind them.
While I didn't get his book in time, Arthur Schwartz "The Food Maven" is another aficionado who has not only written about the region but leads classes and tours there. His web site is a gold mine of tips and recommendations.