Dear aunt mary
I'm writing to express my deep apology for disturb you reminded of my book.
To begin with,you should know that you often borrow books from me,but do not take the initiative to return.In addition,I hold a proverb:"return on time,easier for borrowing next time".Last but not least,I hope you return the book as soon as possible for our friendship.
It would be lighly appreciated if you could read it and give my request in due consideratio.I'm looking forward to your early replay.
Yours sincerely
XXX