I want to suggest that there should be subtags named as a book of sources .
For example, assume we have a subtag named as "finite group theory by Martin Isaacs ".
By that way the questions asked by that book collected in same place which avoid duplicate of questions and make it easy to find asked question as we can numerate them as $3.b.1...$ e.t.c.
In times, by choosing best answers, we will have solution manual of many books.
But I do not know whether this is appropriate for this site or not.
What do you think about this? Can it be useful ?