There are LLM's that can process 1 million token context window. Amazon Nova 2 for one, even though it's definitely not the highest quality model. You just put whole book in context and make LLM answer questions about it. And given the fact that domain is pretty limited, you can just store KV cache for most popular books on SSD, eliminating quite a bit of cost.

You could also fill the context with just the book portion that you've read. That'd be a sure-fire way to fulfill Amazon's "spoiler-free" promise.