The author's purpose for writing the passage is most likely to <u><em>inform, educate or argue</em></u>
The question is incomplete, but i will try my best to give you a general overview that will help you get to the correct answer.
The purpose of an author for writing a book could be to:
Inform
Educate
Argue, etc
Therefore, to find the purpose of an author in a book, you have to first:
Understand the main idea
Check out the supporting details
Identify the moral lesson the author is trying to give. <em>For example</em>, if an author is writing about the dangers of bush burning, then the most likely purpose would be to <em>educate about the dangers of bush burning</em>
