Each book is best at answering questions on its own source material. For now though, it will sometimes answer stuff not related to the book.
The litmus test I use is always asking a new book "Do you know anything about the Chamber of Secrets" -- often times will say no. Sometimes will return the most semantically similar thing it can find.
I posted this earlier but I have a free weekly newsletter for this project. I take book requests from readers, and document the process of building out the library. Readers also get early access to books as they come out.
https://www.konjer.xyz/the-alchemist (disclaimer: built by me)
What specifically is missing from the answers in your opinion?