Super simple plugin that enables indexing of EPUB publications with the SearchWP plugin (https://searchwp.com/).
It is based on Apache Tika (https://tika.apache.org/), so it is necessary that the hosting is enabled to run Java code.
To install (and eventually keep updated) the plugin I suggest to use the very handy Github Updater: https://github.com/afragen/github-updater
Last but not least: in order to upload EPUB files as Media in WordPress, you first need to enable mime-type: https://gist.github.com/gregoriopellegrino/895add2f01746733c5da2a5969079470.