The Constant Princess is the story of Princess Catalina of Spain, daughter of King Ferdinand and Queen Isabella. The plot centers on the treaty arrangement between Spain and England, a promise to stand together against France in the exchange for Princess Catalina's marriage to Prince Arthur, son of Henry VII. I read this book following The White Queen & The Red Queen. It worked out perfectly. Though written prior to the Cousins' War books, chronologically the story falls just 15 years after the end of the stories told in both books. Philippa Gregory takes some license with the historical details, but mainly remains true to the historical facts that have been gathered. Without spoiling any of the plot, it is easiest to just say that she hypothesizes on things that were not wholly understood. This book was a great read and I read it fairly quickly. The only thing that I did not like was the way the book ended rather abruptly. The saving grace is that, having read other books by Ms. Gregory, I know that the story again picks up with the plot of The Other Boleyn Girl. I don't think I will ever be bored with her books.