I'm writing to express my deep apologies for disturbing you and reminding you of my book.

First of all, you should know that you often borrow books from me, but you don't return them voluntarily. Besides, I also hold a proverb: "Return it on time, and it will be easier to borrow it next time". Last but not least, for the sake of our friendship, I hope you will return the book as soon as possible.

I would appreciate it if you could read and give due consideration to my request. I look forward to your early replay.

yours respectfully/sincerely

XXX