I have NZBgeek and Tabula Rasa setup with readarr, but it seems to struggle to find most books. Any recommendations for indexers that handles eBooks better?
Yeah, from my understanding the more indexers the better. If you want to just try some out, I know the usenet community on Reddit has a list of some of the free indexers. They typically have a daily limit but not a bad place to start if you're just getting into usenet. Also if you do a lot of downloading, you should check out the arr software. In my experience it works really well with usenet.