I'm currently reading The Lord of the Rings: A Reader's Companion by Wayne Hammond and Christina Scull, and I was wondering if questions related to this book, which is obviously about LotR, would be considered "on topic".
As this is a non-fiction book about a fantasy universe, questions about the subject matter ("why does Wayne Hammond think this") probably would be on topic.
By comparison, questions about the book itself or the author would probably be off-topic.
Per my normal advice, why not visit the chatroom and pre-ask the question?
Books like this would fall under one or both of these entries in our site's scope:
- Historical or societal context of a work
- Behind-the-scenes and fandom information
So I'd expect most questions on these sorts of books about fantasy (or sci-fi!) works to be on topic, as their focus would overlap with these areas.