This a new, excellent method of searching the content of ebooks:

Combining e-publishing with the Internet makes sense given Google’s indexing (and probably ranking) of EPUB content decision. The fact that it took eight years to accomplish should be the only unexpected element. The update mentions without further information that EPUB file format was included to Google’s indexable file types on their documentation.

EPUB File Structured Format

Based on a standard created by the International Digital publication Forum, EPUB is an XML-based eBook publication format that was subsequently combined with the World Wide Wide Web Consortium (W3C). The merger aimed to unite electronic book publishing with the Internet so that they can enhance one another.

You can read more in an article by Roger Montti published in the searchenginejournal.com web site at: https://bit.ly/3x5QdSy.