The Exchange of Princesses: A Novel;

Set in the French and Spanish courts of the eighteenth century, this novel is based on a true story about the fate of two young princesses caught in the intrigues and secrets of the moment
 
Philippe d’Orléans, the regent of France, has a gangrenous heart—the result of a life of debauchery, alcohol, power, and flattery. One morning in 1721, he decides to marry eleven-year-old Louis XV to the daughter of Philippe V of Spain, who is only four. Orléans hopes this will tie his kingdom to Spain. But were Louis to die without begetting an heir—the likeliness of which is greatly increased by having a child bride—Orléans himself would finally be king. Orléans tosses his own daughter into the bargain, the twelve-year-old Mlle de Montpensier, who will marry the Prince of Asturias, the heir to the Spanish throne.
 
The Spanish court enthusiastically agrees and arrangements are made. The two nations trade their princesses in a grand ceremony in 1722, making bonds that should end the historical conflict. Nothing turns out as expected.