For those of you looking for a good place to buy books (archaeology and other subjects) on Naples, the Campania region and the south of Italy in general - may I suggest Libreria Neapolis.
This tiny bookshop is located right in the heart of Naples, just off Spaccanapoli. In fact, it's conveniently located on S. Gregorio Armeno for those who have just visited Underground Naples and want to pick up a book before buying a nativity scene.
Admittedly it's the size of a small broom cupboard, but they manage to pack in a lot of interesting books into that space, including a small archaeology section.
The key to success though is using them to order what you're looking for, as they are incredibly happy to source the most obscure Pompeian tome - although they are obviously much better at dealing with Italian publications.
Their website is difficult to browse, but you can get round this by using their name in Google along with some decent key words - you'll find lots fo books come up in their thematically organised lists.
If you don't find what you're looking for - ask!
Via San Gregorio Armeno, 4 Napoli - Italia
ph./fax: +39 (0)81 5514337