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.
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?