If this looks hard to install (you need to host a server, supabase, docker, lots of python dependancies) - instead, you could just install this to Edge https://microsoftedge.microsoft.com/addons/detail/epubreader... to use it's built in Copilot sidebar to chat about the book. It would work identical to this project AFAICT, except the back end would be Microsoft.
Thank you for sharing the information/comments. The extensions you have presented are indeed great, but they seem to lack features such as full text search, page count progress tracking, text highlighting, notes, etc. Also, regarding RAG, I find it a bit cumbersome as I have to import the epub every time I open the book. I am not familiar with this tool, so sorry if I am wrong.