因遗失借书致歉 Because of losing a borrowed book
I am terribly sorry to tell you that I have lost the valuable book you were so kind to lend me last week. I read it everyday and intended to finish it next month. Last night when I came to my room, it was nowhere to be found. I will try to recover it as soon as possible. If I fail to find it, I will get a new book for you.
But I am afraid it can never take the place of the old one. Old books are like old friends. Once lost, they can never be replaced. They are connected with cherished associations which the new ones can never have. And for this irrecoverable loss, I am to blame. I was so careless with my things. This is a warning to me to be more careful in the future.