I am terribly sorry to tell you that I have lost the valuable book you were so kind to lend me last week.

很抱歉，上星期您借给我的那本书竟遗失了。

youdao

I am very sorry to tell you that I've lost the book you lent me.

我将你借给我的那本书弄丢了，实在很抱歉。

youdao

I am very sorry to tell you that I've lost the book you lent me.

我将你借给我的那本书弄丢了，实在很抱歉。

youdao