Easiest part first (I can just quote myself!):
"Unanswerable" is not a problem.
If it's truly unanswerable, then it's provably unanswerable and proving it should make for a good, solid answer. Which makes it answerable in the Stack Exchange sense of the concept.
If it's not provably unanswerable, then it shouldn't be closed as unanswerable for what I hope are obvious reasons.
Now, let's tackle the other bits.
"Triva" is not trivial.
We used to have a close reason called "Too Localized." It's gone now, because our Benevolent Stack Overlords came to understand that "too localized" wasn't the problem with those questions; it's even useful to have questions about very specific situations! What we thought was a localization problem turned out to be a collection of problems that are common to those sorts of questions, but were actually already covered by other close reasons.
Trivia questions occupy a similar position. The problems with trivia questions are already enshrined in other close reasons. Just like with "too localized," we should not be passing value judgements on the usefulness of questions when deciding if they're on topic. (I know of a handful of sites that have waaay more traffic than we do, which take a hardline stance on questions which don't "have a problem." We don't have enough traffic to need that kind of weeding-out, and would have trouble identifying non-trivia with a similar gauge anyway given the nature of our topic, so I don't think this is a practical measure for a close reason on lit.se.)
(NB: Trivial questions are arguably down-voteable because they Do Not Show Research; trivial questions border on that frightening notion of "common knowledge," for which there is not room in this margin. As always, up/down vote with your conscience. Voting to close is a completely different mechanic.)
The XKCD decision does not set precedent for this case.
Whether one agrees with that meta's decision or not, it's specifically addressing questions which ask about a non-book thing which is mentioned in a book (comic). That is not what's happening with the Asimov chair-of-books question, so we can't treat its conclusion as fait accompli for this new question. But it IS useful to look at the principle it invokes and see how that applies here:
The meta's reasoning is based on the broad principle that we should limit our topicality to things which are reasonably answered by an expert on literature, which I agree with. So far we've only had a discussion about applying that principle to non-book stuff mentioned in books; we haven't had a discussion about how that principle applies to authors' lives, publicity and marketing, etc... until now!
I personally don't think that principle cuts those subjects out of lit.se, which brings me to the last part:
"Literary life" is applicable here.
The context of the photo is necessarily and immediately connected to Asimov's identity as a writer. If Asimov ever posed on a throne of his own publications, that's a statement about his prolific output and was probably a publicity stunt organized by his publisher. Either of those--making a statement about his own literary efforts OR participating in a marketing gimmick--would make it clearly part of his life as an author and reasonable for an expert on books to be aware of or have the specialized skills/resources to uncover.