In an earlier review for FileSearchy commenter smaragdus wondered out loud if there are any utilities that support searching in epub (and fb2) files. Indeed the search for such a tool seems to confirm that it is a rare feature. I stumbled upon dnGREP, however.
dnGREP searches inside most text files and some like pdf require a plugin which come with the program and reside in the program’s folder. It integrates with the right click menu, can search plain text, Word documents, inside archives and more. It wasn’t clear at first if the latter includes epub files, then I found