Separating a book

There are many reasons for separating a book into parts. But I think it is mostly used to either change the perspective or changing the subject. For Cry, the Beloved Country, I think that Paton uses three separate books for changing the subject and introducing a new idea or character. It also helps emphasize the important things for the previous book because you know that if Alan Paton brings up the same concept as before, it is probably something that you should pay attention to.