It’s difficult to tell to what extent books are encoded into the model. The data might be there in some abstract form or another.
This is a court case so the accusers are going to have to prove it.
The evidence provided is that ChatGPT can produce two-page summaries of the books. The summaries are of unknown accuracy, I haven't read the books myself so I have no idea how much of those summaries are hallucinations. This is very weak.