Finding eBooks (.epub) via content on my Mac I have a lot of eBooks on my Mac. But whereas I can find any PDF on my Mac by searching their content, this doesn't work with Spotlight when applied to .epub files. 
(The only clumsy way of doing this is putting all the eBooks onto google drive. But the results are frustrating. the eBooks are treated somehow like "Package Contents" and only the pertaining page of the book, not the book are found.) 

Anyhow, is there a (simple) way to find eBooks by content?
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.