[write date here]
Dear [principal's name},
Subject: Torn and damaged book found in library
I am writing this letter to inform you that while I was in the library, I encountered a book that I knew should be reported. It was all torn up and horribly damaged. The book's title is __________. I request that this book is replaced and the person who damaged it should be held responsible. Thank you for listening.
Sincerely,
[your name]