I've not used it, but isn't this kind of what NotebookLM does?

You drag a source into it such as a books PDF and then you have a discussion with it.

https://notebooklm.google

What I’m imagining is an LLM that is strongly tied to the book, in the sense of being RLHF’d for the book (or something along those lines), like an author able to cater to any reader interested in the book, but also confined to what the book is about. An LLM embodiment and generalization of the book. Not an LLM you can talk about anything where you just happen to talk about some random book now. The LLM should be clearly specific to the book. LLMs for different books would feel as distinct from each other as the books do, and you couldn’t prompt-engineer the LLM to go out of the context the book.