I have seen people asking for book recommendations. But as it is opinion based it is removed. Then what is the use of the tag book-recommendation?.
I'm not certain if the tag should be removed (and such questions declared off-topic), but I do feel that we need to work on quality control for these questions and answers. Compare our current laissez-faire attitude about "reference requests" to physics.SE's policy.
So I don't think that reference (book) requests are by definition "primarily opinion based", though I do agree that many that have been posted so far are phrased in such a way. It might be more productive to start to cull the bad ones (or at least lock them "for historical reasons") and develop a policy about both questions and answers of this sort.
(Incidentally, this is something that's been on my mind since just before the winter break, but for which I do not have time to devote right now. If someone agrees with me and wants to be more proactive in starting to formulate a policy about reference requests, that would be great!)