Could someone explain to me, why rare books are so precious for them?

Say i feed the largest LLM a book of an alien civilisation, that it definitely hasn't seen before. Then this tiny piece of text muds the vast ocean (latent space) of the model minimally. It will not be able to cite from that book reliably after that fine tuning. Especially for rare books, because them being rare implies, that there aren't 1000s of other books, that encode the same information.

It's general language modelling capabilities might get an iota better, of course. But for putting factual information into it, wouldn't RAG be a much more solid approach?