Q# is Microsoft's programming language for working with quantum computers. Are questions that deal with the language itself (e.g. How do I initilaize a Qubit to a given value in Q#) that don't deal with quantum computing generally considered on topic?
They relate to both computers and quantum mechanics, and I can't really think of any other criteria for on-topicness.
(This answer is a bit brief, but it's really all I think I need to say.)
It would presumably be on-topic on SO too, but I feel like this is just as good if not better of a place for those questions. I'm not versed in Q#, but I feel like the majority of users on SO aren't experts in quantum computing the same way users of this site are likely to be, and that would have a large effect on the answers.
See other sites, like Science Fiction and Fantasy vs Literature, where some books are on-topic for both.