【正确答案】
Dear Henry,
I am sorry to tell you that I am unable to return the book that you were kind enough to lend me last month.
I finished the book last week, and intended to return it to you today as I had promised to do. Yesterday I was unable to find it in the bookcase, where it should have been. Obviously someone has taken it away without first letting me know.
I am still trying to search the book. Even if I finally recover the book, my delay in returning it to you will certainly have caused you some inconvenience.
Again I do apologize for not returning the book. I'll try to get a new one for you if I still couldn't find it within one week.