Philippa Gregory's story is as ambitious as its main character, Catalina, iron-willed daughter of Spanish crusaders King Ferdinand and Queen Isabella. But The Constant Princess lacks the charm of its heroine, who, through determination, patience, and sex appeal, wins her way into England's royal court in the 16th century. ''I will be Queen of England. That is my destiny,'' Catalina repeatedly reminds herself, her ladies-in-waiting, and anyone else who will listen. Alas, Gregory's abrupt changes in narration from first to second person are disconcerting, and the book's messy framework fails to do justice to the majestic subject.