After reading the article about Preston’s book, I found this video of him talking about the book.(In English, although his Spanish is incredible) There’s about 10 minutes of introductions, but near the beginning he answers the question that a journalist once asked him which was what he thought his career was about. (Spoilers: basically to overturn the misconception that Franco was not a good Cristian gentleman, among other things).
Also, an interesting quote when talking about the book was “This is not about Spaniards, this is about human beings. All of these things would have happened in any country where those circumstances prevailed”.