How am I to understand the writer of a book when it has been stated the first part of the book is presented in the third person and the last part is written in the first person?
Writing in thre third person describes what happens to someone.
e.g. "She crossed the room and opened the door."
In the first person the writer describes what he or she (the writer) did.
e.g." I crossed the room and opened the door."
Most books are written in the third person singular, but if writers want to describe their own experiences, they write in the first person singular.
Sometimes a writer will change from first to third or vice versa to achieve a particular effect or just to make the book a bit different or special.