I am not for or against. But I want to state a point:
I think we should keep in mind that we can refer people to go read the textbook and understand it and solve their problem in place of asking on CS.SE if we push the idea of closing because there is a general reference.
For a reference question to really work when closing a question it should be really easy for the OP of the question to read the reference question and its answers and understand how to use them to solve their own question. If it is not so then it really does not answer the OP's question, so it probably shouldn't be closed as duplicate. If someone has read the reference and still has not got the answer to their question then I am not sure if it is right to close the question as duplicate. Ideally the OP should explain (either in their initial posting or by an edit after closing) why the reference does not answer their question.
I think the more important criteria is if the question is going to be useful for others or just specific for the particular user and of no future use.