I had borrowed your book, [Book name], on [Day/Date]. I am sorry about losing it. I am aware this is extremely immature behavior, and I regret showing it. Please forgive me for this mistake.
It was unintentional. [Explain the reason]. I did not realize I had lost it till it was too late. I am sorry for the worry and inconvenience this has caused you. I am willing to compensate for this loss. Please let me know how I can make up for it.
I look forward to your consideration, and I hope we can put this incident aside.
Sample Apology Letter For Academic Misconduct
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Apology Letter Not To Repeat Mistakes
Apology Letter After Cheating
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Letter To Principal Apologizing For Your Irregularity In The College
Apology Letter To Wife For Hurting Her
Apology Letter Asking For Forgiveness
Apology for Missing Event