I;m trying to understand what you mean by every sentence. Apparently you have questions on how these sentences were corrected.
I have only understood the first one.
We say a book by Agatha Christie because it is Passive Voice - it is implied: a book that was written by Agatha Christie
The third question would be correct if it were While I am in Toronto next week I hope to see Tom. WHILE means during your staying in Toronto, you hope to see....
As far as the second question is concerned, could you please send the whole sentence as it has been corrected ?
I hope I have been helpful