I prefer LazyLibrarian over Readarr but it still leaves a lot to be desired for end-user usability. One of the big issues with ebooks is that data is a mess, with each book having a billion different editions with spotty metadata support that makes it hard to tell what is what.
Goodreads seems like it was a decent source of data for these types of projects but they shut off new API access a couple years ago and legacy access can go away at any moment. Hardcover seems like a promising API alternative but not sure if anyone has started integrating with them yet. Manga and comics seem to be in a better state, with a more rabid fanbase maintaining data but still nowhere near what’s available for movies and tv.