Hello. You did not refer to which book this question refers to, which makes it impossible for it to be answered, but I will try to help you in the best possible way.
It is only possible to know, in fact, the reason for a book to be divided into parts if that book is read. However, generally, books are divided into parts to show a passage of time and to show a change in perspective, narrator or narrative focus on the story. However, the biggest reason for dividing a book into parts is to show great changes that will occur in history, or that have already occurred.