Definitely *not* an Marbled White (Melanargia galathea). Given your location, this is probably a Spanish Marbled White (Melanargia ines). See
http://www.leps.it/SpeciesPages/MelanInes.htm.
I'd recommend Tristan Lanfranchis' book "Butterflies of Europe", which gives excellent guidance on the distinguishing features of European Butterflies. Although it's listed at
http://www.ukbutterflies.co.uk/books.php, this will link to Amazon which will take ages since it's not in stock.
Any visitors know how to contact Tristan directly (I've lost Tristan's email address when my previous computer died)?
Cheers,
- Pete