I found a website with extensive information on beaches, clubs and resorts in Spain:
home page translated to English:
lugaresnaturistas.org Our Favorite Nudist Beachesfor Spanish, French or Portuguese:
lugaresnaturistas.orgIt may be an advertising-based commercial site, but it appears legitimate and very informative.
The site also has a few very interesting items in a section semi-translated as "Subjects Monográficos" (i.e. written works focusing on a particular subject.) That section includes a web book,
Documents on Naturism, with chapters on social attitudes toward naturism, the history of naturism in Spain, etc.
I suspect a lot of the material is lifted from elsewhere, but it is convenient to have it assembled and presented in an organized manner.