[GERGE]

Under macOS or Linux it is pretty easy. Create this bash script and make it executable:

#!/bin/bash
PAT=${1:?"Usage: grep-epub PAT *.epub files to grep"}
shift
: ${1:?"Need epub files to grep"}
for i in $* ;do
  echo $0 $i
  unzip -p $i "*.htm*" "*.xml" "*.opf"   |
    perl -lpe 's![<][^>]{1,200}?[>]!!g;' |
    grep -Pinaso  ".{0,60}$PAT.{0,60}"   |
    grep -Pi --color "$PAT"
done

Put it somewhere in your home and link it to /usr/local/bin, this is how I did it:

sudo ln -s ~/Apps/CLI/grep-epub.sh /usr/local/bin/grep-epub

Then just use it like this:

To find all occurences of sea in Dubliners:

grep-epub "sea" Dubliners.epub

To find only the word sea"

grep-epub " sea " Dubliners.epub

To count, just use it with wc, GNU word count utility.

In Windows, you can use this with Linux subsystem.