【答案解析】Dear William,
I am terribly sorry to tell you that! 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 buy a new one for you.
But I am afraid it can never take the place of the old one. And for this irrecoverable loss, I am to blame, for I was so careless with my things. This is a warning to me to be more careful in the future.