I can't find anything to do this - anyone know of a shell extension to do it.
I found this ==>>
xplorer˛ blog: search filter for EPUB ebooks
IMO it misrepresents itself. It's not a keyword search tool (i.e. on dc:subject), it's a content search tool via an iFilter. Unfortunately it doesn't display EPUB text in File Manager Preview, despite what it says on the tin.
FWIW - I just searched a folder of a 100K+ moderate length epubs for 'credit default swaps' in the blink of an eye. The results match those from a search of equivalent DOCXs with Windows and X1.
I mean Preview the text - not the covers.
BR