The series started as Pérez-Reverte was disappointed with the lack of treatment of the history of the Spanish Golden Age in the school textbook of his teenaged daughter Carlota. He commissioned Carlota to gather documentation for him (hence, she is billed as co-author of the first novel [not in the US]) and developed the stories.
Pop Quiz: What do Johnny Depp, Viggo Mortensen and Kate Beckinsale have in common?