Books that are currently in print, or have been recently enough that if you can't find them in a book shop, you should be able to find them in a library.

The books in the first part of the list are specifically about Spanish food. 

The books in the second part of the list include a fair amount of information about Spanish foods, but deal with other cultures (primarily Italy) as well as Spain. Professor Ken Albala (Dept of History, University of the Pacific) has several books in this category.