Simplify the following text for a Grade 9 native English-speaking person.
Original: Prescott 's studies initially remained broad , but he started preparing material on Ferdinand and Isabella in January 1826 .
Simplified: Prescott decided that he wanted to write a book about Ferdinand and Isabella , who were King and Queen of Spain in the early 1500s , in January 1826 . After gathering source material , Prescott started writing the History of Ferdinand and Isabella in October 1829 .