The new "Expert Intelligence" feature allows you to bring titles from Google Play Books directly into Gemini Notebook, which means you can ask questions about the material, as well as generate plans, infographics, AI podcasts, and more based on their contents.

Google's AI note-taking app now allows you to interact with books | The Verge
https://www.theverge.com/tech/985567/google-gemini-notebook-expert-sources-books

Ah, I was hoping from that it was about some new integration with Google Books. Anything that I can download as a PDF can be included in a notebook as long as it is less than 500,000 words or 200MB, so that includes most books on Google Books. But I was basically hoping for exactly what they did, but for Google Books, not Google P[l]ay books

https://elephas.app/blog/notebooklm-source-limits

That covers most books.