For a writer, histories about kings and battles have one major disadvantage: they're just about useless. They can give a historical scaffolding, a sense of time so you don't mix up your history into an anachronism mish-mash, but they are notoriously dull because they lack living detail.
Fortunately, this is not a book about Spain. It is not even a book about the Spanish Crown. It's about the members of the royal family. On both sides. Since this starts with how the future Queen Isabel was born, the Pragmatic Sanction revoking the Salic law, and the king's brother refusing to accept it, starting the Carlist claim that lasted through out the period.
I like the stories best. Even if some would be difficult to pilfer, and all of them would be implausible in a novel.
The king's brother persuades the king that the Pragmatic Sanction will cause bloodshed, and so he revokes it on his deathbed. Except that when the man comes to prepare the corpse, he finds that the king is not dead yet. (The king took many, many, many steps to ensure it stuck in the remaining year of his life.)
A woman warns the king that his prospective bride has already borne a child. This king, however, has buried three wives without having living children. A bride who can bear children!
An abdicated queen insists on receiving ex-Empress Eugenie with full ceremony -- going to the door to greet her despite the cold and her pneumonia. She explains that ex-royalty are very sensitive to slights.
The queen dowager is dying. Her daughter-in-law, the queen, goes to wake her daughters. Their first question is whether the revolution started.
A queen in mourning, weeping over her dead husband. A servant is reluctant to intrude, but must announce the ambassador whose credential she must receive as regent. When he does so, she instantly transformed to a regal and powerful woman.
A child-king insists on his tutor going ahead of him into a room, because here he is merely a pupil.
A Spanish princess marries into Louis-Phillippe's family. She doesn't like it. How badly? Well, when she is escaping his overthrow, after their carriage was nearly invaded by a mob, when she is walking through a muddy town, late in the evening, her reaction was that at least it wasn't sitting about sewing.
The Carlists, facing the dreadful issue that with the extinction of male-line descendents of the king's brother, the rightful male-line successor is -- the current king. So they quickly fudge up an illegitimate descent for him so they can support someone else.
And many more.