"Even if" is an odd phrasing. Why would you expect that to be possible? LLMs don't have direct access to their training set. Having a book in the training set just means they emit text in the style of the book - it's not like a database they can query. But they can summarize very well, if the text is front of them.
I mean, currently you have to wait for a human to summarize a book, before you can read an llm rephrasing of that summary. Or you have to buy the book and pass it to the llm employing some tricks to avoid reunning out of context length.
