Finding eBooks (.epub) via content on my Mac
Here's an open source plugin you can use to do what you're asking.

https://github.com/jaketmp/ePub-quicklook

Simply follow the installation instructions on their page and according to the description, it will then index it for use with Spotlight. There's also a plugin for Quick Look support on the same page. To download a tested version, go to "Releases" and get the latest version.

At the time of writing this, that is version 1.7.