Stephen,
I don't have a definative answer for you but late in 2007 found us at Camping Salata in Roses on the Costa Brava.
Its a very nice site that is used by all nationalities both as a base for a holiday but also as a stopping off point up and down the Costas'. But I degress.
It would have been at the beginning of November and as the week went on I noticed that various caravans were being 'uprooted' if thats the right word from their pitches. It was evident that these caravans had been there for the season by the amount of 'gear' surrounding them, the age of the caravan and the haphazard way in which the 'vans were being packed. The owners discarded anything that didn't fit through the door and this property seemed to be pounced upon by everyone at the rubbish disposal area. (I'm talking about chairs, windbreaks, bodyboards etc).
By the end of the week, the campsite was a lot emptier and only occupied by foreign nationals and not 'locals'.
So it would be right that you can get a seasonal pitch in Spain but you will have to make your own enquiries at a particular campsite as I doubt that there is a book that would guide you.
Happy searching