I'm aware the questions about the value of a chess set are off-topic, however I'm wondering if questions about book values are on-topic. Specifically, I'm curious about why a certain book tends to be somewhat difficult to find, and is priced so high.
I think it's a fine question, as long as you don't ask people for (an estimate of) the value of the book, which is primarily opinion-based (not off-topic, but it will be closed anyway). Incidentally, there's no reason why this shouldn't apply for chess sets, chess clocks and chess computers as well.
The close reason 'primarily opinion-based' reads:
Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise.
(emphasis mine). I think this could be one of those 'good questions' the reason refers to.
You can ask about the general valuation of chess books in the context of chess, it is probably a valid question. But these days a strong chess engine with a group of humans (seconds) to help with the interpretation of engine evaluations, should render most books unnecessary or obsolete.
Also, similar to computer books, chess books are in a niche market with limited value as their content becomes outdated quickly (reduced information value), even publishers are increasingly switching to ebooks which are sold for a lower price.