If you know Spanish,or can read it, the selection is enormous.
There are not that many books on the Spanish royal family written in English. Theo Aronson, the late and great, wrote ROYAL VENDETTA _ which tells you the story of rthe schism between the Carlist and Isabeline lines. Jon van der Kiste's book on the Spanish Royal House tells you the basic information one needs to know.
THE GOTHA – Still a Continental Royal Family, Volume 3, will contain a rather longish chapter on the Spanish royals.
In my magazine, THE EUROPEAN ROYAL HISTORY JOURNAL, there have been quite a few articles about various branches and members of the Spanish royal family as well.
Best, Arturo Beéche